Prior to this commit, the different `R.call_tir*` variations would
wrap the arguments into an in-line `relax.Tuple`, if it is not
already a `relax.Tuple`.  While this allows a tensor to be passed into
these functions as a single argument (`R.call_tir(func, arg, ...)`
instead of `R.call_tir(func, [arg], ...)`), the wrapped Relax variable
may already refer to a tuple.

This use of a variable to refer to an argument tuple rather than an
in-line argument tuple is not allowed by Relax.  (See discussion on
apache#15916 for details.)  However, by
wrapping a variable `args: R.Tuple(R.Tensor, R.Tensor, ...)` into a
tuple-of-tuples, the error occurs after the expression has already
been generated, and refers to an expression `R.Tuple(R.Tuple(R.Tensor,
R.Tensor, ...))` that doesn't appear anywhere in the user's input.
This can make debugging difficult (see
apache#17239 for an example).

This commit updates the argument-handling in `R.call_tir` to only
generate an in-line `relax.Tuple` if the arguments do not already have
`relax.TupleStructInfo`.  If the argument was provided as a Relax
variable bound to a tuple of arguments, it will still produce an
error.  However, that error will occur much earlier, and will
explicitly state that the argument must be a `relax.Tuple` instead of
a `relax.Var`.