Specify the operand data type constraints of operations (#646)

* Specify the operand data type constraints of operations

Introduce constraints for input operands, either directly (e.g.
input's dataType can only be "float32" or "float16") or indirectly
(e.g. weight's dataType must be the same as input's).

Fixes #283

* gruCell: bundle hiddenState in with other type validations

* Identity should accept all types

* Add reduceMean restriction

* Update gemm to check c data type too

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Co-authored-by: Dwayne Robinson <[email protected]>