Allow dynamic padding in preprocessor

[jbischof]

Currently our preprocessors require a static sequence length for all batches (example). However, on average max sequence length in a batch can be much smaller than max sequence length possible. Choosing a smaller static sequence_length can give speedups but affects accuracy.

Let's allow a "dynamic" option where each batch is padded to the minimum amount possible. This would likely interfere with XLA compilation so this would have to be scoped and benchmarked.