Avoid double key lookup on callback.py (#3018)
Avoid double key lookup on callback.py (#3018)

On method on_epoch_end, to add new keys to the history dict, first it is
verified if a key is not on the history dict and if that is the case, a new key
is created on the history dict with an empty list as value.

However, this operation search for a key twice in the dict. This same behavior
can be achieved in a single step using dict setdefault method.
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