Add distributed implementation for understand sentiment LSTM model

[abhinavarora]

Fixes #7749

[putcn]

error while running this test:

Traceback (most recent call last):
  File "notest_understand_sentiment_lstm_dist.py", line 187, in <module>
    main()
  File "notest_understand_sentiment_lstm_dist.py", line 131, in main
    dict_dim=dict_dim, class_dim=class_dim)
  File "notest_understand_sentiment_lstm_dist.py", line 73, in lstm_net
    emb = fluid.layers.transpose(x=emb, axis=[1, 0, 2])
TypeError: transpose() got an unexpected keyword argument 'axis'

Added commits to fix the issue

[abhinavarora]

@putcn Thank you for pointing that out. It seems the method signature was updated recently. I have fixed the issue in commit aaaf995