[Quant] Add get_symmetric_qnnpack_qconfig_mapping (#87002)
Summary: Today, in order to get XNNPACK quantized ops to work,
the user must write some code that refers to private data
structures (`_FIXED_QPARAMS_OP_TO_OBSERVER`) to create a
QConfigMapping that is compatible with the symmetric constraints
in the QNNPACK BackendConfig. This is because
`get_default_qconfig("qnnpack")` produces a QConfig that does
not satisfy these constraints, and the default QConfigMapping
for QNNPACK uses this Qconfig.
Instead, we simply put this code into a helper function to make
it easier for the user to run XNNPACK quantized ops. In the
future, once there is feature parity between the set of ops
supported by QNNPACK and XNNPACK, we should revisit whether
to simply change `get_default_qconfig("qnnpack")` to return
an XNNPACK-compatible QConfig.
Test Plan:
python test/test_quantization.py
TestQuantizeFx.test_symmetric_qnnpack_qconfig_mapping
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87002
Approved by: https://github.com/vkuzo
