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.

PiperOrigin-RevId: 650386711