Remove duplicate feed dict constructing

[chunyang-wen]

in python/ray/experimental/sgd/sgd_worker.py

    def compute_gradients(self):
        start = time.time()
        feed_dict = self._grad_feed_dict()
        # Aggregate feed dicts for each model on this worker.
        for model in self.models:
            feed_dict.update(model.get_feed_dict())

with _grad_feed_dict definitions:

    def _grad_feed_dict(self):
        # Aggregate feed dicts for each model on this worker.
        feed_dict = {}
        for model in self.models:
            feed_dict.update(model.get_feed_dict())
        return feed_dict