Multipack processor type checking method

[jennyzhang-petuum]

Is your feature request related to a problem? Please describe.
Implement type checking for multipack processor.

Describe the solution you'd like
As for expected_types_and_attributes funciton, when checking the type, we need to make sure input packs has either one of them. And record perhaps we could include all the generated types because the later processor could get all these types.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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