Improve inferability of shape::Dims for cat (#39294)
`cat` is often called with Varargs or heterogenous inputs,
and inference almost always fails. Even when all the arrays
are of the same type, if the number of varargs isn't known
inference typically fails. The culprit is probably #36454.
This reduces the number of failures considerably, by avoiding
creation of vararg length tuples in the shape-inference pipeline.
(cherry picked from commit 815076b392821609815d5d77077e042ef08d2e14)
