Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43086
This PR changes the format of `ConvPackedParam` in a nearly backwards-compatible way:
* a new format is introduced which has more flexibility and a lower on-disk size
* custom pickle functions are added to `ConvPackedParams` which know how to load the old format
* the custom pickle functions are **not** BC because the output type of `__getstate__` has changed. We expect this to be acceptable as no user flows are actually broken (loading a v1 model with v2 code works), which is why we whitelist the failure.
Test plan (TODO finalize):
```
// adhoc testing of saving v1 and loading in v2: https://gist.github.com/vkuzo/f3616c5de1b3109cb2a1f504feed69be
// test that loading models with v1 conv params format works and leads to the same numerics
python test/test_quantization.py TestSerialization.test_conv2d_graph
python test/test_quantization.py TestSerialization.test_conv2d_nobias_graph
// test that saving and loading models with v2 conv params format works and leads to same numerics
python test/test_quantization.py TestSerialization.test_conv2d_graph_v2
python test/test_quantization.py TestSerialization.test_conv2d_nobias_graph_v2
// TODO before land:
// test numerics for a real model
// test legacy ONNX path
```
Note: this is a newer copy of https://github.com/pytorch/pytorch/pull/40003
Test Plan: Imported from OSS
Reviewed By: dreiss
Differential Revision: D23347832
Pulled By: vkuzo
fbshipit-source-id: 06bbe4666421ebad25dc54004c3b49a481d3cc92
