Don't truncate leading 1s if they are unbacked (#95141)
This prevents us from guarding on leading unbacked SymInts.
The previous attempt at https://github.com/pytorch/pytorch/pull/94521 I got the logic a bit wrong. My idea there was to avoid slicing when the values to be set have low enough dimensionality that they definitely aren't too long. To do this, I need to compute the difference between the data to be set, and the post-slice space for the values. But I incorrectly compared against the *pre-slice* space in the original PR. Another version of this PR which is wrong is to compare against variableIndices.size(); but remember that in advanced indexing with tensors/lists, each of the individual indices specify what coordinates to read out of each dimension! A third incorrect attempt tested `variableIndices[0].dim()`, which is only correct if you don't broadcast one of the later variable indices, and if there are enough variableIndices to cover all dims. This is all quite complicated, so I went for a simpler solution of checking if the leading dim had a hint before testing if it is not equal to one.
BTW, there is no test for this one stripping behavior. There is now a test for this, based off the real code that caused the problem.
Signed-off-by: Edward Z. Yang <ezyangmeta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95141
Approved by: https://github.com/ngimel
