Factor backend routing logic out of convolution forward (#67790)
Summary:
This PR introduces a new function `_select_conv_backend` that returns a `ConvBackend` enum representing the selected backend for a given set of convolution inputs and params.
The function and enum are exposed to python for testing purposes through `torch/csrc/Module.cpp` (please let me know if there's a better place to do this).
A new set of tests validates that the correct backend is selected for several sets of inputs + params. Some backends aren't tested yet:
* nnpack (for mobile)
* xnnpack (for mobile)
* winograd 3x3 (for mobile)
Some flowcharts for reference:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67790
Reviewed By: zou3519
Differential Revision: D32280878
Pulled By: jbschlosser
fbshipit-source-id: 0ce55174f470f65c9b5345b9980cf12251f3abbb
