Fix issue 49

[srikarplus]

The following error has been addressed in this patch

File "/home/venv/lib/python3.7/site-packages/fastprogress/fastprogress.py", line 264, in add_child
self.child.prefix = f'Epoch {self.main_bar.last_v+1}/{self.main_bar.total} :'
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

Looks like self.last_v has been wrongly initialized to None.

Addressed the issue #49

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[sgugger]

Thanks for your PR.

No, this is not a mistake, this has the meaning that on_iter_begin should be called if iterating through the progress bar again, as is addressed here.

Issue #49 does not contain a reproducer and was filed before the code was widely refactored. If there is still a bug like this, I would need a reproducer. This fix will actually break the main behavior.

[srikarplus]

Ohh my bad!!! I thought since the variable self.last_v was already initialized to 0 here, again initializing to None was not required. Thanks for taking some time to explain @sgugger