Fixed two long-standing bugs in knowledge distillation:
 * Distillation loss needs to be scaled by T^2 (#122)
 * Use tensor.clone instead of new_tensor when caching student logits (#234)
Updated example results and uploaded the script to generate them

This script shows how to specify a compression-schedule directly
using Distiller's API, instead of using a YAML specification

examples/scheduling_api/direct_api_pruning.py

* Move image classification specific setup code to separate script at
  examples/classifier_compression/ptq_lapq.py
* Make ptq_coordinate_search function completely independent of
  command line arguments
* Change LAPQ command line args function to update existing
  pre-existing parser (changed CLAs perfix to 'lapq' for more clarity)
* Enable LAPQ from compress_classifier.py (trigger with --qe-lapq)
* Add pointers in documentation

Convert Distiller PTQ models to "native" PyTorch PTQ (#458)

* New API: distiller.quantization.convert_distiller_ptq_model_to_pytorch()
* Can also be called from PostTrainLinearQuantizer instance:
    quantizer.convert_to_pytorch()
* Can also trigger from command line in image classification sample
* Can save/load converted modules via apputils.load/save_checkpoint
* Added Jupyter notebook tutorial

* Converted modules have only the absolutely necessary quant-dequant
  operations. For a fully quantized model, this means just quantization
  of model input and de-quantization of model output. If a user keeps
  specific internal layers in FP32, quant-dequant operations are added
  as needed
* Can configure either 'fbgemm' or 'qnnpack' backend. For 'fbgemm' we
  take care of preventing overflows (aka "reduce_range" in the PyTorch
  API)

"Loss Aware Post-Training Quantization" (Nahshan et al., 2019)

Paper: https://arxiv.org/abs/1911.07190 
Reference implementation:
  https://github.com/ynahshan/nn-quantization-pytorch/tree/master/lapq

Proper documentation is still TODO, for now see the example YAML file
at 'examples/quantization/post_train_quant/resnet18_imagenet_post_train_lapq.yaml'

* Implemented in distiller/quantization/ptq_coordinate_search.py
* At the moment that file both the model-independent algorithm
  implementation and image-classification specific sample script.
  Still TODO: Refactor that

* Post train quantization changes (range_linear):
  * Added getters/setters for quantization parameters (scale/zero_point)
    and clipping values
  * Add option to save backup of FP32 weights to allow re-quantization
    after quantizer was created.
  * Add option to clip weights in addition to activations
  * Fix fusions to not occur only when activations aren't quantized
  * RangeLinearFakeQuantWrapper:
    * Make inputs quantization optional
    * In case of ReLU + ACIQ, clip according to input stats

* Data loaders:
  * Add option to not load train set at all from disk (to speed up
    loading time in post-training runs)
  * Modified "image_classifier.py" accordingly

(we use 8-bit values below, but this applies to any bit-width)
* We use the notion of "full" and "restricted" quantized range for
  symmetric quantization (see section 2.2 in https://arxiv.org/abs/1806.08342)
* "Full" quantized range ==> [-128, 127], "restircted" ==> [-127, 127]
* Until now, when doing symmetric quantization we assumed a "full"
  range when saturating after quantization, but calculated the scale
  factor as if the range was restricted. This means we weren't making
  full utilization of the quantized range.
* On the other hand, in some other implementations of quantization (e.g.
  TensorFlow), the "restricted" range is used.
* So, we make it an option to use either the proper "full" range
  (q_min = -128) or "restricted" range (q_min = -127).
* LinearQuantMode.SYMMETRIC now means the "full" range is used, and
  added LinearQuantMode.SYMMETRIC_RESTRICTED for using the "restricted"
  range.
* Updated tests and documentation.

In PostTrainLinearQuantizer and QuantAwareTrainRangeLinearQuantizer

* Make it easier to find sample apps for different workload types
* Add READMEs for sample apps the didn't have any
* Update readmes with experiment results where applicable

Added tables of results for 85% sparsity

* Weights-only PTQ:
  * Allow RangeLinearQuantWrapper to accept num_bits_acts = None, in
    which case it'll act as a simple pass-through during forward
  * In RangeLinearQuantParamLayerWrapper, if bits_activations is None
    and num_bits_params > 0, Perform quant and de-quant of the
    parameters instead of just quant.
* Activations-only PTQ:
  * Enable activations only quantization for conv/linear modules. When
    PostTrainLinearQuantizer detects # bits != None for activations 
    and # bits == None for weights, a fake-quantization wrapper will
    be used.
* Allow passing 0 in the `--qe-bits-acts` and `--qe-bits-wts` command
  line arguments to invoke weights/activations-only quantization,
  respectively.
* Minor refactoring for clarity in PostTrainLinearQuantizer's replace_*
  functions

Results changed following commit 9e7ef987 (#402)

compute-summary and png-summary currently work with image classifiers
only.

When multi-processing, we want only one process to generate the
summary, while the other processes do nothing (lazy bums!)

Add an example of compressing OD pytorch models.

In this example we compress torchvision's object detection models - FasterRCNN / MaskRCNN / KeypointRCNN.
We've modified the reference code for object detection to allow easy compression scheduling with YAML configuration.

- define ALMOST_ONE
- define op_type
- remove sanity assert (need to understand what tolerance value to use
in the assert)

Co-authored-by: csc12138
Co-authored-by: wangyidong3

- define ALMOST_ONE
- define op_type
- remove sanity assert (need to understand what tolerance value to use
in the assert)

* Previous implementation:
  * Stats collection required a separate run with `-qe-calibration`.
  * Specifying `--quantize-eval` without `--qe-stats-file` triggered
    dynamic quantization.
  * Running with `--quantize-eval --qe-calibration <num>` only ran
    stats collection and ignored --quantize-eval.

* New implementation:
  * Running `--quantize-eval --qe-calibration <num>` will now 
    perform stats collection according to the calibration flag,
    and then quantize the model with the collected stats (and
    run evaluation).
  * Specifying `--quantize-eval` without `--qe-stats-file` will
    trigger the same flow as in the bullet above, as if 
    `--qe-calibration 0.05` was used (i.e. 5% of the test set will
    be used for stats).
  * Added new flag: `--qe-dynamic`. From now, to do dynamic 
    quantization, need to explicitly run:
    `--quantize-eval --qe-dynamic`
  * As before, can still run `--qe-calibration` without 
    `--quantize-eval` to perform "stand-alone" stats collection
  * The following flags, which all represent different ways to
    control creation of stats or use of existing stats, are now
    mutually exclusive:
    `--qe-calibration`, `-qe-stats-file`, `--qe-dynamic`,
    `--qe-config-file`

* pruning: add an option to virtually fold BN into Conv2D for ranking

PruningPolicy can be configured using a new control argument fold_batchnorm: when set to `True`, the weights of BatchNorm modules are folded into the weights of Conv-2D modules (if Conv2D->BN edges exist in the model graph).  Each weights filter is attenuated using a different pair of (gamma, beta) coefficients, so `fold_batchnorm` is relevant for fine-grained and filter-ranking pruning methods.  We attenuate using the running values of the mean and variance, as is done in quantization.
This control argument is only supported for Conv-2D modules (i.e. other convolution operation variants and Linear operations are not supported).
e.g.:
policies:
  - pruner:
      instance_name : low_pruner
      args:
        fold_batchnorm: True
    starting_epoch: 0
    ending_epoch: 30
    frequency: 2

* AGP: non-functional refactoring

distiller/pruning/automated_gradual_pruner.py – change `prune_to_target_sparsity`
to `_set_param_mask_by_sparsity_target`, which is a more appropriate function
name as we don’t really prune in this function

* Simplify GEMM weights input-channel ranking logic

Ranking weight-matrices by input channels is similar to ranking 4D
Conv weights by input channels, so there is no need for duplicate logic.

distiller/pruning/ranked_structures_pruner.py
-change `prune_to_target_sparsity` to `_set_param_mask_by_sparsity_target`,
which is a more appropriate function name as we don’t really prune in this
function
-remove the code handling ranking of matrix rows

distiller/norms.py – remove rank_cols.

distiller/thresholding.py – in expand_binary_map treat `channels` group_type
the same as the `cols` group_type when dealing with 2D weights

* AGP: add example of ranking filters with virtual BN-folding

Also update resnet20 AGP examples

Fix the EE code so that it works with the current 'master' branch,
and add a test for high-level EE regression

Co-authored-by: Bar <29775567+barrh@users.noreply.github.com>
Co-authored-by: Guy Jacob <guy.jacob@intel.com>

* Add blacklist to quantizer. In PTQ put Dropout on the blacklist.
* Update notebooks to use 2-phase stats collection
* Other small fixes

Force loading on the CPU which always has more memory than a
single GPU.  This is useful for models that cannot be loaded onto
a single GPU.

As documented in issue #395, some of the command-line examples in the
AMC notebooks are incorrect.
Also, fix some bugs that were introduced with the refactoring of the
low-level pruning API

* Greedy search script for post-training quantization settings
  * Iterates over each layer in the model in order. For each layer,
    checks a user-defined set of quantization settings and chooses
    the best one based on validation accuracy
  * Provided sample that searches for best activations-clipping
    mode per layer, on image classification models

* Proper handling of mixed-quantization settings in post-train quant:
  * By default, the quantization settings for each layer apply only
    to output quantization
  * Propagate quantization settings for activations tensors through
    the model during execution
  * For non-quantized inputs to layers that require quantized inputs,
    fall-back to quantizing according to the settings used for the
    output
  * In addition, provide mechanism to override inputs quantization
    settings via the YAML configuration file
  * By default all modules are quantized now. For module types that
    don't have a dedicated quantized implementation, "fake"
    quantization is performed

* Misc. Changes
  * Fuse ReLU/ReLU6 to predecessor during post-training quantization
  * Fixes to ACIQ clipping in the half-range case

Co-authored-by: Lev Zlotnik <lev.zlotnik@intel.com>
Co-authored-by: Guy Jacob <guy.jacob@intel.com>

Some refactoring of the low-level pruning API

Added distiller/norms.py - for calculating norms of various sub-tensors.

ranked_structures_pruner.py:
-Removed l1_magnitude, l2_magnitude. Use instead distiller.norms.l1_norm
-Lots of refactoring
-replaced LpRankedStructureParameterPruner.ch_binary_map_to_mask with
distiller.thresholding.expand_binary_map
-FMReconstructionChannelPruner.rank_and_prune_channels used L2-norm
by default and now uses L1-norm (i.e.magnitude_fn=l2_magnitude was
replaced with magnitude_fn=distiller.norms.l1_norm)

thresholding.py:
-Delegated lots of the work to the new norms.py.
-Removed support for 4D (entire convolution layers) since that has not been
maintained for a longtime. This may break some old scripts that remove entire
layers.
-added expand_binary_map() explicitly so others can use it. Might need to
move to a different file
-removed threshold_policy()

utils.py:
-use distiller.norms.xxx for sparsity stats