Gradient flowing between inference_cell and generator_cell

[Rustleman]

There seems to be an issue in the inference_rnn function where the inference_cell and generator_cell are connected together:

tf-gqn/gqn/gqn_draw.py

Lines 417 to 421 in bc84f24

	# estimate statistics and sample state from posterior
	mu_q, sigma_q, z_q = compute_eta_and_sample_z(inf_state.lstm.h,
	scope="Sample_eta_q")
	# input into generator RNN
	gen_input = _GeneratorCellInput(representations, query_poses, z_q)

It looks like the gradient flows through z_q.

Adding the line z_q = tf.stop_gradient(z_q)
seems to improve the results when just the generator_rnn is used during testing.