add option to set a constant random seed for deterministic results

[eyaler]

seems that the random seed always involves the system clock and i could not find a way to set a constant seed to get deterministic results for debugging. (also, please let me know if i am opening too many issues. maybe we should have a mailing list or forum?)

[ppwwyyxx]

Just added a function utils.fix_rng_seed in 546af8b.
You'll also need to avoid any kind of parallelism:

1. set 1 inter/intra op parallelism thread in session config
2. don't use any kind of prefetch
3. set use_locking=True in the optimizer

And set seed at the beginning of your program by both fix_rng_seed and tf.set_random_seed.

And this still cannot guarantee deterministic results, they just make things more deterministic. After doing the above things I see very tiny differences when training mnist for a couple of epochs.
Because a lot of common ops (both on GPU and CPU) are non-deterministic. tensorflow/tensorflow#3103