Add CTC models

[csukuangfj]

Here are the TODO lists. Help from the community is appreciated.
@pkufool please have a look which part(s) you are interested in.

• Merge Merge the branch v2.0-pre to master k2#1085 once all CI tests pass
• Move https://github.com/k2-fsa/k2/blob/master/k2/csrc/torch_api.h to https://github.com/k2-fsa/k2/tree/master/k2/torch/csrc
• Remove the dependency kaldifeat_core from k2_torch. We can make the binaries depend on kaldifeat_core directly.
• Export the library k2_torch in https://github.com/k2-fsa/k2/blob/c3a7404e0d9da4d8d99f3f15c54d80b321c1b960/cmake/k2Config.cmake.in#L48
• Add the following functions to torch_api.h
  • A function to load HLG.pt. The returned type can be std::shared_ptr<k2::FsaClass>. We can define an alias FsaClassPtr for it, like RaggedShapePtr
  • A function to wrap k2::CtcTopo(). It should also return a value of type FsaClassPtr.
  • A function for CTC/HLG decoding. It takes the following inputs:
    - log_softmax_out: a 3-D tensor of shape (N, T, C)
    - log_softmax_out_lens, a 1-D tensor of shape (N,)
    - FsaClassPtr, can be either a CtcTopo or an HLG
    and it returns std::vector<std::vector<int32_t>>
• Once the above items are done, we can add CTC decoding to sherpa
  • Add support for conformer_ctc models from icefall. The following two links are very helpful
    - https://github.com/k2-fsa/k2/blob/v2.0-pre/k2/torch/bin/ctc_decode.cu
    - https://github.com/k2-fsa/k2/blob/v2.0-pre/k2/torch/bin/hlg_decode.cu
  • Add support for conformer ctc models from wenet. The following link is helpful:
    • [Not for merge] Add full-chunk mode CTC decoding for models from WeNet k2#872
      We can use https://github.com/wenet-e2e/wenet/releases/download/v2.0.1/chs.tar.gz and https://github.com/wenet-e2e/wenet/releases/download/v2.0.1/en.tar.gz for testing.
      I suggest that we also add scripts to show how to build an HLG graph for models from wenet. We can put the scripts
      and models on huggingface.
• Support streaming CTC decoding.
• Add documentation for CTC decoding.

[csukuangfj]

Note: We need to add both C++ and Python support for CTC decoding.

[pkufool]

OK, thanks! Shall we wait for 1 day, if no one volunteer from the community I can do the first five from tomorrow.

[csukuangfj]

OK, thanks! Shall we wait for 1 day, if no one volunteer from the community I can do the first five from tomorrow.

Yes, sure.

[pkufool]

Since no volunteers, I will take over this work.