[TorchToTosa] Refactoring to separate construction of legal/illegal ops and conversion patterns.

[sahas3]

This PR refactors TorchToTosa to separate the construction of legal/illegal ops and conversion patterns in their own functions:

1. populateTorchToTosaConversionLegalOps -- populate any ops that are legal after the conversion pass
2. populateTorchToTosaConversionIllegalOps -- populate any ops that are illegal after the conversion pass
3. populateTorchToTosaConversionPatterns -- populate the ops conversion patterns

Currently the (il)legality of the ops that are (il)legal after the conversion pass runs is embedded within the conversion pattern. Our end goal is to write a new pass pipeline that converts torch ops to a mix of tosa, linalg, tensor, etc dialect ops. The reason we want to also emit tosa ops (instead of using the existing TorchToLinalg to emit linalg+tensor+...) is because some operations like conv2d encodes the padding behavior in the op in tosa unlike the linalg version -- this helps in lowering the tosa.conv2d to a custom implementation that does padding on the fly.

To implement this new pipeline we need to be able to separate out the illegal tosa ops from the conversion pattern itself. Otherwise we will hit an issue for ops like AtenMaxDimOp which can be lowered to both tosa and linalg + others dialects. Not all AtenMaxDimOp can be lowered successfully to tosa as the implementation uses tosa.reshape which cannot handle multiple dynamic dimensions but the TorchToLinalg lowering can handle it. In the current behavior the pipeline will stop as soon as the existing TorchToTosa conversion runs as AtenMaxDimOp will be marked as an illegal op.

Essentially we want to be able to control what the legality of the ops should be independent of the conversion pattern. This is also inline with the conversion patterns in the llvm-mlir repo such as https://github.com/llvm/llvm-project/blob/000e790be35b77a01872851646d54432a203542c/mlir/lib/Conversion/SCFToControlFlow/SCFToControlFlow.cpp#L718

"THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY."