X-entropy assert probabilties sum to approximately one
The CI build failed with 
```
ERROR: The project you're trying to build requires Bazel 6.1.0 (specified in /workspace/.bazelversion), but it wasn't found in /usr/local/lib/bazel/bin.
```

which doesn't seem related to my change? @gbaned what are the next steps here? > @gbaned what are the next steps here?

Hi @beyarkay Sorry for the delay. It looks like your PR relates to the Keras component. Please submit it to the github.com/keras-team/keras repository instead. Thank you for your contribution! 
@fchollet, @qlzh727 This seems like something that should have been mentioned in CONTRIBUTING.md 😞 
Currently, it's easy enough to forget that `from_logits` is false by default and it's easy to pass in logits when cross-entropy is expecting probabilities. This won't raise an error, because the values are scaled by [this line](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/keras/backend.py#L4891).

This change adds an assertion to check that the sum isn't *too* far from summing to 1