Support the general constant folding pass in OneFlow MLIR

[terryzhao127]

Folding all operations involving only constants which are represented as nn.Parameters.

• [x] Make a constant-like VariableOp for folding constants. (VariableWithoutCtrlOp)
• [x] Add pattern rewrites for converting VariableOp to VariableWithoutCtrlOp.
• [ ] Solidating VariableWithoutCtrlOp by reading corresponding data from local files.
• [ ] Folding specific operations on VariableWithoutCtrlOp.
• [ ] Generalize the folding pass for all operations involved.

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