Update shape operator inference for opset 15 with start and end attributes

[royudit]

Description

Fixes a bug in symbolic shape inference class where shape operator inference was done without using start and end attributes for opset 15 or greater.

Motivation and Context

• Symbolic shape inference using dynamic axes and torch export with dynamo fails as shape operator infer function.
• The shape operator in its actual implementation returns the full shape tensor (ignoring the start and end attributes) leading to failed auto merging of dimensions emerging from the _merge_symbols function.
• This modification/bugfix makes the infer shape function compatible with pytorch dynamo export graph and the resulting model can be quantized successfully.

[royudit]

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