reshuffle_each_iteration args now default to True

[kr-ish]

The documentation for the tf.Dataset.data.shuffle function states the following:

• reshuffle_each_iteration: (Optional.) A boolean, which if true indicates that the dataset should be pseudorandomly reshuffled each time it is iterated over. (Defaults to True.)

However, the default value in the function is None:

def shuffle(self, buffer_size, seed=None, reshuffle_each_iteration=None):

The function calls the ShuffleDataset class, whose __init__ function also sets the same argument to None by default, and uses the following logic to set the default value of the argument to True:

if reshuffle_each_iteration is None:
  self._reshuffle_each_iteration = True
else:
  self._reshuffle_each_iteration = reshuffle_each_iteration

This commit sets the argument to True by default in both the function and the class, making the above code block redundant and replacing it with only self._reshuffle_each_iteration = reshuffle_each_iteration.

[mrry]

Thanks for the contribution, but unfortunately I am not going to accept this PR for a few reasons:

• It would be a breaking change for any existing code that calls Dataset.shuffle(..., reshuffle_each_iteration=None).
• In general I prefer None for default arguments, because it makes it easier to write wrappers around an API. If the default argument is set to True, it's difficult for a wrapper to use the default of the wrapped function, without making a copy of the default at each layer of wrapping. Using None and implementing the default at the innermost level is simpler to maintain.
• Using None is also consistent with the necessary style when the default argument can be a mutable type (like a list) and it is unsafe to put the default value in the argument list.

[kr-ish]

Thank you for reviewing this! I appreciate the details- using None makes sense to me now. Thanks again!