Make CTC work with more encoder-only models

Summary:
Changes include:
1. Added get_normalized_probabilities to the encoder-only base class FairseqEncoderModel
2. Made CTCCriterion work for both batch_first (LSTMSubsampleEncoderModel) and batch_second (LSTMEncoderOnly) encoder types
3. Added tests for different encoder and CTC combinations.

TODO:
CTC still doesn't work for VGGLSTMEncoderModel so I have disabled that. Will debug and send out fix in another diff.

Reviewed By: jay-mahadeokar

Differential Revision: D15158818

fbshipit-source-id: acb484bad705c937d676d2c3dcde3e3562d68ed9