Fix `dtype` and `assign*` in `AutocastVariable`.

[copybara-service[bot]]

Fix dtype and assign* in AutocastVariable.

The dtype property would return to true dtype of the variable, instead of the dtype of the value that you get explicitly via .value() or implicitly by doing any operation.

This would cause seemingly correct things like this to fail with a dtype mismatch:

y = variable * tf.cast(x, variable.dtype)

Forcing users to write workarounds like:

v = variable.value()
y = variable * tf.cast(x, v.dtype)

Additionally, assign, assign_add, assign_sub expected the value to be of the true dtype, not the cast dtype.

This would cause seemingly correct things like this to fail with a dtype mismatch:

variable.assign(variable * factor)

(This is a common use case for non-trainable variables.)

Forcing users to write workarounds like:

variable.assign(tf.cast(variable * factor, variable.dtype))

This changes fixes these issues to make autocasting fully transparent:

• dtype returns the cast dtype if applicable
• assign* accept the cast dtype for the value if applicable

Note that this is consistent with how autocasting works in Keras 3.