Fix None Type error while using MultiHeadAttention

[GCS-ZHN]

This PR is modified based on previous PR #165 by @cainmagi ,

Main change features:

• automatical detect and filter not array like elements in forward output list/tuple/dict. For example, MultiHeadAttention module return a tuple which contain a NoneType value as a placeholder of attention weight.
• If filtered output contain no element, raise a ValueError to notify user instead of original NoneType AtrributeError.
• Replace -1 to batch_size in dict/list/tuple output shape because I believe it will be more properly.