Rename 'wf' to 'samples' in AudioEncoder

[Dan-Flores]

This PR renames variables in the C++ code and Python from 'wf' to 'samples' for clarity.

[NicolasHug]

Thanks for the PR @Dan-Flores !

There seem to be 2 remaining failing job:

FAILED test/test_encoders.py::TestAudioEncoder::test_num_channels[to_file-2-2] - RuntimeError: Could not open input file: /tmp/pytest-of-root/pytest-0/test_num_channels_to_file_2_2_0/output.mp3 Invalid data found when processing input

I see this one on main as well, so it's probably not related to this PR. I'll try to figure out what's going on separately.

There's a build error on the macos job:

/Users/ec2-user/runner/_work/torchcodec/torchcodec/pytorch/torchcodec/src/torchcodec/_core/./../../../src/torchcodec/_core/Encoder.h:54:7: error: private field 'desiredNumChannels_' is not used [-Werror,-Wunused-private-field]
  int desiredNumChannels_ = -1;

I think you just need to remove the correspdoning line, which probably is an artifact of a previous merge.

After that this is good to go!

[NicolasHug]

Thanks for the PR @Dan-Flores !