Support passing JActivity from python to java

[zhichao-li]

What changes were proposed in this pull request?

This PR would enable us to pass JActivity from Python to Java which is consist of ndarray or nested list of ndarray . JActivity(Python) --> JActivity(Java)
i.e:

        value = np.random.random_sample([2, 3])
        nested = JActivity([ [value, value],  [value, value]])
        back = callBigDlFunc("float", "testPyToJavaActivity",
                             nested)

How was this patch tested?

unittest

[zhichao-li]

@jason-dai How about this style?
By default, we only return JActivity as JavaObject without Serialization and we can use toPy method to enforce the serialization to convert it to ndarray or nested list of ndarray.

Java Side:

  def testPyToJavaActivity(jactivity: JActivity): JActivity = {
    jactivity
  }

Python Side:

        back = callBigDlFunc("float", "testActivityWithTableOfTable")  # back is a JActivity from Java which is a JavaObject without serialization
        assert isinstance(back, JavaObject)
        back_again = callBigDlFunc("float", "testPyToJavaActivity", back)  # back_again is also a JavaObject withou serialization
        assert isinstance(back_again, JavaObject)
        back_again = back_again.toPy()
        assert isinstance(back_again.value, list)
        assert isinstance(back_again.value[0], list)
        assert isinstance(back_again.value[0][0], np.ndarray)

[zhichao-li]

jenkins passed

[jason-dai]

Please check with @dding3 on how this will be used?

[dding3]

I usually set state which returned from get_hidden_state directly in set_hidden_state. And I tried this PR with simplernn/lstm/multirnncell, it works good.

[jason-dai]

Why do you need to do that in python @dding3 ? I think this is needed in seq2seq training, and that's done in scala side?

[dding3]

Yes, it's done in scala side.

However in scala Recurrent, we support get/setHiddenState and they are public APIs. Besides, both tf and torch support user set init state. I am thinking it would be better if we support these functionality in python as well.

[jason-dai]

I don't think we need to add that at this moment if no one is expected use that.

[zhichao-li]

BTW, the there's no overhead for get/set JActivity if without calling toPy() on this PR.