Add transformer model

[k-w-w]

Just a direct copy from the working repo - no changes have been made.
edit: Made one edit to update the licensing message. No more edits in this PR!

[karmel]

Thanks

[karmel]

Ah, linting. I guess you will have to fix the lint errors before it goes in; hopefully they are minimal.

[k-w-w]

@karmel the lint tests fails with the description: "Internal CI infrastructure error. Please apply label 'kokoro:force-run' to rerun the build.". The test lints all the files without error. Is it ok to ignore the internal CI build, and merge this PR?

[karmel]

That kokoro message is just information about how it is configured. Errors are:

************* Module official.transformer.model.beam_search
C:452, 2: Consider using enumerate instead of iterating with range and len (consider-using-enumerate)

************* Module official.transformer.model.transformer
C:316, 2: Missing method docstring (missing-docstring)
C:360, 2: Missing method docstring (missing-docstring)

************* Module official.transformer.model.model_utils
C:111, 0: Trailing newlines (trailing-newlines)

************* Module official.transformer.model.ffn_layer
C: 74, 0: Trailing newlines (trailing-newlines)
C: 39, 2: Missing method docstring (missing-docstring)

[k-w-w]

@karmel oooh I see. I've pushed the fix to these lints