fix error message on specialization with Dim.DYNAMIC (#155738)
Previously specialization error messages would render sources that were pretty far from source-code names. E.g., given args named `x, y, zs`, the source for `y.size()[0]` would be rendered as `args[0][1].size()[0]`.
This is because we created artificial local names following `(args, kwargs)` structure instead of reusing signatures. This PR fixes that situation.
Basically we map prefixes of key paths that correspond to original arg names to root sources corresponding to those names; the rest of the key paths hang from these root sources.
Differential Revision: [D76461391](https://our.internmc.facebook.com/intern/diff/D76461391/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/155738
Approved by: https://github.com/bobrenjc93
