*   Added a short doc explaining how `Estimator`s save checkpoints.
    *   Add documentation for ops supported by the `tf2xla` bridge.
    *   Fix minor typos in the doc of `SpaceToDepth` and `DepthToSpace`.
    *   Updated documentation comments in `mfcc_mel_filterbank.h` and `mfcc.h`
        to clarify that the input domain is squared magnitude spectra and the