Add explanation option to Chex shape asserts so that error messages can bring in lots of contextual information and make failures much more actionable.

[copybara-service]

Add explanation option to Chex shape asserts so that error messages can bring in lots of contextual information and make failures much more actionable.

Example usage:

def _assert_explanation() -> str:
      return (f"{targets.shape=} {encoded.shape=} {_shape(decoder_mask)=} "
              f"{_shape(encoder_decoder_mask)=} {_shape(logit_mask)} "
              f"{enable_dropout=} {decode=} {max_decode_length=} {prefill=} "
              f"{prefill_lengths=}")

chex.assert_shape(x, init_targets.shape, _assert_explanation)

This allows a couple of nice usages, especially in complicated functions/classes:

• Define an informative error message once (e.g. just below the function entry point)
• Use repeatedly throughout the function with convenient syntax.

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