Handle scalar tensor shape formatting

[fallintoplace]

Summary

Fixes scalar tensor shape formatting in tensor_shape_to_c_string_impl().

The helper used to write an opening parenthesis, skip the dimension loop for scalar shapes, then patch p - 2 and p - 1. For an empty shape, p - 2 points before the output buffer. Scalar shapes now return "()" before entering the trailing-comma rewrite path.

Also adds tests for:

• scalar shape {}
• rank-1 shape {3}
• negative dimensions preserving the existing ERR formatting
• a shape near kTensorDimensionLimit

Test Plan

• git diff --check -- runtime/core/exec_aten/util/tensor_shape_to_c_string.cpp runtime/core/exec_aten/util/test/tensor_shape_to_c_string_test.cpp
• direct c++ smoke test against tensor_shape_to_c_string.cpp covering scalar, rank-1, negative, and near-limit shapes

I also attempted the CMake/gtest path, but a fresh root configure entered the broader project codegen flow and did not complete promptly in this local checkout.

[pytorch-bot]

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[fallintoplace]

@pytorchbot label "release notes: none"