update multistep_optimizer for tensorflow gpu

[AgoloCuongHoang]

I found by using the original class MultistepAdamOptimizer, I got a problem of invalidArgumentError: Cannot assign a device for operation (see details below). This problem is only for tensorflow-gpu. On tensorflow it works fine. I did a lot of investigation on this (e.g. try adding normal soft placement tf.Session(config=tf.ConfigProto(allow_soft_placement=True, log_device_placement=True)) - the trick people recommended without success).

After investigation I found two actions needed to fix this issue. 1. Convert int to float, and 2. The class inherits directly from optimizer.Optimizer, not AdamOptimizer. With this I found it solved the issue. Note that without any of these twos it will not work.

Let me know if you have any question regarding to this pull request. Meanwhile let me know if you need the code to replicate the issue of invalidArgumentError. Feel free if you think you could find a better solution.

Finally, I tagged @fstahlberg as well as he wrote the original MultistepAdamOptimizer class so that he will aware of this issue.

Thank you!

---

A subpiece of the error:

Traceback (most recent call last):
  File "/home/cuong.hoang/anaconda2/envs/py36_env_tensor_gpu_pip_local/lib/python3.6/site-packages/tensorflow_core/python/client/session.py", line 1365, in _do_call
    return fn(*args)
  File "/home/cuong.hoang/anaconda2/envs/py36_env_tensor_gpu_pip_local/lib/python3.6/site-packages/tensorflow_core/python/client/session.py", line 1348, in _run_fn
    self._extend_graph()
  File "/home/cuong.hoang/anaconda2/envs/py36_env_tensor_gpu_pip_local/lib/python3.6/site-packages/tensorflow_core/python/client/session.py", line 1388, in _extend_graph
    tf_session.ExtendSession(self._session)
tensorflow.python.framework.errors_impl.InvalidArgumentError: Cannot assign a device for operation training/beta1_power/IsInitialized/VarIsInitializedOp: Could not satisfy explicit device specification '' because the node {{colocation_node training/beta1_power/IsInitialized/VarIsInitializedOp}} was colocated with a group of nodes that required incompatible device '/job:localhost/replica:0/task:0/device:GPU:0'. All available devices [/job:localhost/replica:0/task:0/device:CPU:0, /job:localhost/replica:0/task:0/device:XLA_CPU:0, /job:localhost/replica:0/task:0/device:XLA_GPU:0, /job:localhost/replica:0/task:0/device:GPU:0]. 

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[AgoloCuongHoang]

@googlebot I signed it!

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[afrozenator]

Hi @AgoloCuongHoang -- Thanks for the nice investigation, but Travis seems to report that the tensor2tensor/utils/multistep_optimizer_test.py test failed, -- the logs are here, can you investigate?

https://travis-ci.org/tensorflow/tensor2tensor/jobs/626315428?utm_medium=notification&utm_source=github_status

[afrozenator]

To make Travis trigger - I will close and reopen the pull request.

@lukaszkaiser - Do you have thoughts on this? I think changing MultistepAdamOptimizer to not inherit from AdamOptimizer adds a lot of code, whatever the root cause is, in theory it shouldn't be because of the inheritance itself I feel. But obviously after much work the root cause wasn't found, but a fix made.

[afrozenator]

It seems like Travis did run and fail - https://travis-ci.org/tensorflow/tensor2tensor/jobs/627551248?utm_medium=notification&utm_source=github_status

So there is no need to do the close and reopen dance.

[AgoloCuongHoang]

@afrozenator: Sorry for my late response. I have some important work to do recently.

I just fixed the file and I think it passed the test (multistep_optimizer_test.py). It however does not pass some certain task that I indeed have no ideas why it is (I never touched on them I believe).
What could I do next?

[AgoloCuongHoang]

Please see my latest commit which fixes the issue and passes the multistep_optimizer_test.py

[AgoloCuongHoang]

@afrozenator: Any update on this? To be clear I am OK if the pull request reject was rejected. I am just curious what was going on. Thx.

[lukaszkaiser]

@AgoloCuongHoang : could you make this new optimizer in a separate file and in a separate class? Just so we have the old one too for compatibility with old code?

[AgoloCuongHoang]

@lukaszkaiser: Done. Let me know if you need me to do any thing further

[afrozenator]

Thanks a lot @AgoloCuongHoang merging this now!