Add RNN Transducer Loss for CPU

[vincentqb]

This pull request introduces rnnt_loss and RNNTLoss as a prototype in torchaudio.prototype.transducer using HawkAaron's warp-transducer.

• The python interface remains currently the same as the original.
• This has been tested as integrated within ESPNet here.

Follow-up work detailed in #1240.

cc @astaff, internal, #1099

[cpuhrsch]

I think the documentation could be improved a bit. It could also be useful to reference the paper.

[mthrok]

This will get installed in global namespace, outside of torchaudio package directory.
Please put it in torchaudio package.

[mthrok]

If torch is not found, shouldn't it be failing?

[cpuhrsch]

If you don't want to copy-paste the docs from the functional you could reference it here within the documentation.

[vincentqb]

Some TODOs:

• Use TORCH_CHECK to raise an error instead of writing to standard output, here, e.g. used here.
• Investigate the memory allocation for workspace, here.

Some follow-ups:

• Move libsox to a third_party subfolder as suggested above.
• Investigate using AT_DISPATCH_FLOATING_TYPES.
• Migrate the checks to C++.
• Add GPU implementation and compilation.
• Patch the submodule to remove the pytorch deprecation warnings.
• Refactor, see internal.

[cpuhrsch]

nit: use the factory function torch.tensor([xyz], dtype=torch.float) instead of the type constructor. Same applies to IntTensor.

[cpuhrsch]

self.assertEqual should be preferred

[vincentqb]

Some more TODOs:

• Remove numpy from tests
• Guard prototype csrc and extension

Some more follow-ups:

• Pass along the DEBUG flag to cmake
• Guard prototype python files by omitting them from torchaudio, see also comment
• Guard building third party transducer even if not added as an extension
• Enable building transducer in nightlies only, not release.
• Build within same folders as libsox, comment and comment
• Remove hardcoded O2/O3 optimization, see comment

Error below also happens on master:

conda_build.exceptions.DependencyNeedsBuildingError: Unsatisfiable dependencies for platform osx-64: {"python[version='>=2.7,<2.8.0a0|>=3.8,<3.9.0a0|>=3.5,<3.6.0a0|>=3.5']", "python[version='>=3.6,<3.7.0a0|>=3.7,<3.8.0a0|>=3.9,<3.10.0a0']", "python_abi[version='3.6.*|3.7.*',build='*_cp36m|*_cp37m']", "python[version='>=2.7,<2.8.0a0|>=3.6,<3.7.0a0|>=3.7,<3.8.0a0|>=3.9,<3.10.0a0|>=3.8,<3.9.0a0|>=3.5,<3.6.0a0']", 'python', 'python_abi=3.9[build=*_cp39]'}

cc comment above

[cpuhrsch]

This could be inlined since it only has one call-site and is pretty small (but that's not the reason to remove an abstraction necessarily).

[mthrok]

@vincentqb Can you define a proper namespace? torchaudio::<something>::rnnt_loss

I am not sure how you want to move on, but if you have a plan to add different type of rnnt, then more descriptive name would work better later, like warprnnt

[mthrok]

Adding anonymous namespace in #1159 for the time being.

[mthrok]

@vincentqb I update the followup description for things addressed in #1159 and #1161. Please stamp these PRs when you have time.

For Enable building transducer in nightlies only, disable for release. I am thinking to add master environment variable that has higher precedence than BUILD_TRANSDUCER, like DISABLE_PROTOTYPE and propagating it from CCI configuration.

[mthrok]

For C++ ABI issue ssee #880