Change the global variable logger to comet_logger

[vince62s]

The point lies here: https://github.com/Unbabel/COMET/blob/master/comet/__init__.py#L22

When we import anything like "from comet import load_from_checkpoint" it will create a global variable "logger" and there are consequences if in the main code calling this import we also have a variable "logger".

Since the likelihood is more "calling comet from another toolkit" rather than the contrary, I would like to rename the logger variable to comet_logger which has to happen only in a few instances.

Do you agree?

[ricardorei]

I agree. I don't see any reason why not.