[Relax] Add CopyWithNewParams interface

[Ubospica]

This is the new PR following #90 after the source branch having been moved to personal repositiory.

This PR adds a new interface relax.utils.copy_with_new_params to copy functions:

def copy_with_new_params(func: Function) -> Function:
    pass

The parameters of the original function would be copied to satisfy the restriction in the well-formed check: any two functions should not share the same parameter variable.

This interface serves for passes that needs to copy some existing function and modify it, such as relax.transform.Gradient.

[SiriusNEO]

I'm thinking if the struct info of params contains Var, should we copy them as well?
For example, the shape of an input Tensor is a relax.Var.

[Ubospica]

I'm thinking if the struct info of params contains Var, should we copy them as well? For example, the shape of an input Tensor is a relax.Var.

@SiriusNEO I perfer not. Because we now only have to satisfy the requirement in well-formed check, which only checks parameter vars of functions. If we wanna copy all vars in the function, we may have to do this.

[MasterJH5574]

I'm thinking if the struct info of params contains Var, should we copy them as well?
For example, the shape of an input Tensor is a relax.Var.

The behavior and semantics of param struct info containing shape var needs more thinking, and currently we have no such example. We will revisit this point after some time and at this moment let’s just go as usual, assuming no shape var appears in the params’ structure info.