[Unity][Pass] BindSymVar pass

[sunggg]

This PR introduces an utility pass that binds symbolic variables to user-provided integer values.
For example, say we have a following IRModule.

@tvm.script.ir_module
class Before:
  @R.function
  def main(
    x: R.Tensor(("m", "n")),
    y: R.Tensor(("m", "n"))
   ) -> R.Tensor(("m", "n")):
      m = T.Var("m", "int64")
      n = T.Var("m", "int64")
      with R.dataflow():
        out = R.matmul(x, y)
        R.output(out)
      return out

We can conveniently bind the symbolic variable by applying After = relax.transform.BindSymVars("main", {"m": 10, "n": 10})(Before).

@tvm.script.ir_module
class After:
  @R.function
  def main(
    x: R.Tensor((10, 10)),
    y: R.Tensor((10, 10))
   ) -> R.Tensor((10, 10)):
      with R.dataflow():
        out = R.matmul(x, y)
        R.output(out)
      return out

This would be useful when specializing shape and providing compile-time shape info (e.g., model params or batch sizes) by eliminating the need to rewrite the model.
cc. @tqchen @psrivas2

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