Include padding mask in generation

[joecummings]

Bug

Expect batched input to match single input e.g.

1. [seq1, ... seq_m] -> generate -> [output1, ...., output_m]
2. [seq1] -> generate -> [output1]

Before this would not create the same output1. The issue was that the src_key_padding_mask was not being propagated forward.

Fix

Create padding mask and add it to model_kwargs and pass it to the forward function.