keras: Avoid unneccesary call to .call() when building models with subclassing.

This fixes a regression in the defun microbenchmarks
(ResNet50Benchmarks.eager_train_with_defun_gpu_batch_32_channels_first etc.)
in tensorflow/contrib/eager/python/examples/resnet50/resnet50_test.py
seen after https://github.com/tensorflow/tensorflow/commit/9a84277be2cb8233c5c14270db6fcdff31ab4d93
(which embeds a model in model)

Without this change, converting a model call to a graph function using
something like:
model.call = tfe.defun(model.call)
could result in redundant nodes being added to the graph function
as the model._set_inputs() call would invoke model.call() again.

PiperOrigin-RevId: 187391494