TFE: Fix bug encountered when using `optimizer.apply_gradients` in a defun.

Prior to this change, `Optimizer` assumed that `not
context.executing_eagerly()` implied that every variable that it was to update
was constructed in a graph. That assumption is incorrect --- TensorFlow
functions can mutate variables captured from or lifted into the eager context. As such, this change removes that assumption.

Fixes #17792

PiperOrigin-RevId: 189633630