List the metrics in the __init__.py

[franchuterivera]

I will be pretty cool if we can do something like:

from autoPyTorch.metrics import  Accuracy, etc..

As of now the metrics are defined individually, like in autoPyTorch/pipeline/components/training/metrics/init.py

The above is useful for the end-user and for unit testing

[ravinkohli]

This can be done now, as all the metrics have been defined in autoPyTorch.metrics.metrics. Is it fine? or should I make them available from autoPyTorch.metrics?

[franchuterivera]

This has been addressed in #52