Track and record hint on SymNode and use when possible (#94201)
Historically, we work out `size_hint` by working it out on the fly by doing a substitution on the sympy expression with the `var_to_val` mapping. With this change, we also maintain the hint directly on SymNode (in `expr._hint`) and use it in lieu of Sympy substitution when it is available (mostly guards on SymInt, etc; in particular, in idiomatic Inductor code, we typically manipulate Sympy expressions directly and so do not have a way to conveniently maintain hints.)
While it's possible this will give us modest performance improvements, this is not the point of this PR; the goal is to make it easier to carefully handle unbacked SymInts, where hints are expected not to be available. You can now easily test if a SymInt is backed or not by checking `symint.node.hint is None`.
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94201
Approved by: https://github.com/voznesenskym
