tensorflow: Change Tensor.__bool__ return from NoReturn to bool

[hoel-bagard]

Closes #13365

This change will remove the false positives currently happening with valid code such as

bool(tf.constant(1))       # True
bool(tf.constant([1]))     # True
bool(tf.constant([[0]]))   # False 

It will however introduce false negatives when using bool on tensors with more than one element:

bool(tf.constant([1, 1]))     # Error

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[hoel-bagard]

@srittau If you have time to have a look.

[srittau]

Thanks! False negatives are preferable to false positives.