Remove lazy behavior from MelScale

[mthrok]

Address #1510

In 0.9.0 we are deprecating the lazy behavior of MelScale because it can make an invalid TorchScript object and it does not align with the design of torchaudio. Now in master branch, we can remove the implementation.

See CONTRIBUTING.md for setting up the development environment.

Steps

1. Replace n_stft: Optional[int] = None with n_stft: int = 201.
2. Remove the branches for n_stft = None
3. Remove __prepare_script__ method
4. Remove the case numel() == 0
5. Remove test

Tests

Run test cd test && pytest torchaudio_unittest.transforms -vk MelScale and make changes to failures.

[Patil2099]

I would love to work on this @mthrok

[mthrok]

Resolved by #1636