Fix GRUCellBlockOp message for invalid rank of x
> LGTM, just reordering the imports back as this would break Copybara otherwise

No problem. Why's that? Internally the imports get rewritten based on where the code is situated in the Google monorepo and the type of the code (OSS -> third_party). Plus, code linters want the imports sorted.
The validation checks that `x` is a matrix, so rank must be 2. ff45913 fixed the crash in https://github.com/tensorflow/tensorflow/issues/58261 but left this typo in an exception message.

Fixes https://github.com/tensorflow/tensorflow/issues/58261

@mihaimaruseac 