Remove _docs.py

[jamarshon]

In #166, we add _docs.py because we can't get the doc string of a class in a @torch.jit.script_method without creating an instance (e.g using the syntax Class.method.__doc__).
So the workaround is to have _docs.py

class Spectrogram:
    forward = torchaudio.transforms.Spectrogram().forward

This creates an instance to expose the docstring to sphinx.
An issue has been made for the pytorch jit pytorch/pytorch#23394