Add mypy Config File

[tomassosorio]

In order to start using Mypy full potential, we must start fixing the issues that he might throw.
Relates to #477

As a starter I add the following definitions to the config file:

allow_redefinition: True
Allows variables to be redefined with an arbitrary type, as long as the redefinition is in the same block and nesting level as the original definition.
ignore_missing_imports: True
Suppresses error messages about imports that cannot be resolved.

[vincentqb]

For reference, pytorch does not have anything beyond python 2.7 compatibility. We do not want python 2.7 of course.

[tomassosorio]

For reference, pytorch does not have anything beyond python 2.7 compatibility. We do not want python 2.7 of course.

@vincentqb I did not understand what you meant by that, mypy currently is not passing at all and a lot of errors occur, this configuration will ignore small details such as a variable being change of its type.
This will allow us to better focus in the errors that occur on torchaudio.

If the decision is to leave mypy default values I will close this PR :)

[vincentqb]

Sorry for the misunderstanding :) My comment was meant as a note to self to reference what pytorch is doing.

Those two change from default seems reasonable to me, and a good start. Thanks for adding this :)

[vincentqb]

LGTM