Refactor to `@tf.function`

[khatchad]

We converted several eager execution function to hybrid execution as part of an assessment of our refactoring tool. The transformations produced were completely automated. We have some preliminary evidence that this improves the run-time performance:

Test	Python version	TensorFlow version	Epochs	Before accuracy	After accuracy	Before loss	After loss	Before elapsed time (s)	After elapsed time (s)	Speedup
MusicTransformer-tensorflow2.0/train.py	3.10.0	2.9.3	5	0.0233113606	0.02381204049	4.956358294	4.923633542	1330.180665	919.3014389	1.446947224

The above analysis was repeated five times for each program version (before and after the refactoring) and the values were averaged. The increased speedup is ~1.45 with seemingly negligible loss of accuracy. We would appreciate any feedback you may have to help us as assess our refactoring approach. Thank you for your time!