Define TORCHAUDIO_API for better visbility control

[mthrok]

Context:

TorchAudio uses dual-binding (PyBind11 and TorchBind) to make custom operations available in Python.
The both binding eventually calls the same implementation contained in libtorchaudio[_XXX].so.
The ones bound via PyBind11 (the ones in torchtext._torchaudio[_XXX]) calls into libtorch[_XXX].so.

(The equivalent illustration from pytorch/text#1806)

This means that libtorchaudio[_XXX].so has to make the symbols (APIs) used by torchtext._torchtaudio[_XXX] visible.

However, the default visibility of symbols in shared libraries are different in Windows.
On Windows all the symbols are by default hidden. To work around this, we use CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS to expose all the symbols.
There is an upper limit of visible symbols one library fine can contain, and this can be problematic in the future.

A better approach is to selectively mark the symbols that should be visible as visible.

Summary of the change set

This commit introduces TORCHAUDIO_API macro which annotates functions with proper visibility.
The core logic was taken from https://github.com/pytorch/pytorch/blob/bcc02769bef1d7b89bec724223284958b7c5b564/c10/macros/Export.h

The behavior is as follow;

For non-Windows: It is always __attribute__((__visibility__("default")))
For Windows:
If the header is included from the compilation unit of libtorchtext, then it resolves to __declspec(dllexport). otherwise it resolves to __declspec(dllimport).

This allows to remove CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS.

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