a) fix shape mismatch when take grad of JITExecutor including Dimshuffle
b) avoid redundant computation in the grad of JITExecutor
c) not pass unused vars as inputs to the grad of JITExecutor to save device memory
d) traverse internal graph only once in JITExecutor ctor instead of traverse
   whole graph in each call of setup_args()
e) expand the gradient graph into the origin graph if all inputs are const

GitOrigin-RevId: ba6a2b29e975c7f63a21785efad87dbda76143d4