message: Fix dtype check in GradientTape
  ResourceVariables handles will have Tensor type as dtype.resource.
  Instead, we should check the dtype of the Variable Tensor.
diff: @@ -957,7 +957,7 @@ class GradientTape(object):
  flat_sources = nest.flatten(sources)
  flat_sources_raw = flat_sources
  flat_sources = [_handle_or_self(x) for x in flat_sources]
  - for t in flat_sources:
  + for t in flat_sources_raw:
  if not t.dtype.is_floating:
  logging.vlog(
  logging.WARN, "The dtype of the source tensor must be "
  