[Quant][fx] Create new FX-based LSTM reference module (#96343)
Summary: The previous LSTM reference module implementation did
not handle dtypes other than quint8 correctly. This is because
the internal LSTM custom module quantization used eager mode,
which did not insert the q-dq ops properly. E.g., we want the
following reference quantized model:
```
[dq -> linear1_fp32 -> q_to_qint32] -> dq -> q_to_quint8 ->
[dq - linear2_fp32 -> q_to_quint8] -> dq -> ...
```
This requires two sets of `q - dq` pairs between two adjacent
ops that have different dtypes (linear1 and linear2). However,
these `q - dq` pairs were not inserted in the old flow, because
eager mode required users to insert Quant/DeQuantStubs manually.
This commit changes the internal LSTM custom module quantization
to use FX graph mode quantization, which automatically inserts
the `q - dq` ops that convert the dtypes between adjacent ops
correctly. However, using FX graph mode quantization here comes
with its own set of challenges that required some hacks to get
the end-to-end flow to work. These hacks are detailed in the
comments in the util functions.
Test Plan:
python test/test_quantization.py TestQuantizeFx.test_static_lstm_with_custom_fixed_qparams
This commit also updates the corresponding test to verify the
dtypes as well as the qparams in the reference quantized graph.
This test case should serve as an example for users to set up
their own LSTM reference module flows.
Reviewers: vkuzo, supriyar, jcaip
Subscribers: vkuzo, supriyar, jcaip
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96343
Approved by: https://github.com/vkuzo
