[TorchToLinalg] Fix AtenReflectionPad2dOp lowering to not assert when dimensions unknown

[giacs-epic]

The asserts make the compiler crash (with a misleading error) also when vertical or horizontal dimensions are unknown (which is a valid state at this stage).
Add dynamic asserts when dimensions are not known.

[zjgarvey]

Makes sense! Does the conversion even work if the input shape is dynamic? Can we add a dynamic test (either a lit test or preferably an e2e test) to make sure that path actually works?

[giacs-epic]

@zjgarvey I just added a dynamic test, it seems to work fine! I also took the chance to add dynamic asserts.

[giacs-epic]

@zjgarvey I just added a dynamic test, it seems to work fine! I also took the chance to add dynamic asserts.

I struggle to understand the CI error. It seems like when exporting to onnx it interprets the input sizes as [1, 1, 1] instead of [1, -1, -1]. A PyTorch bug?