Provide the option to return partial batches

[maxrjones]

Is your feature request related to a problem?

As explained in #82, xbatcher throws away partial batches if the input_dims dimension is not evenly divisible by the value provided.

Describe the solution you'd like

As noted in

xbatcher/xbatcher/tests/test_generators.py

Lines 66 to 67 in f04ad18

	# TODO: decide how to handle bsizes like 15 that don't evenly divide the dimension
	# Should we enforce that each batch size always has to be the same

we could consider adding a parameter that specifies that the batch generator should also return partial batches.

Describe alternatives you've considered

If there is no strong need for this feature, we could keep the status-quo of only returning equal-sized batches.

Additional context

No response