Move tensor_shape_to_c_string back to runtime/core

[swolchok]

I previously moved it to the wrong place because of build issues. Had to couple this with replacing sizes_to_string so that the buck2->cmake system would place it in executorch_core instead of portable_kernels and not break the build.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> Move tensor_shape_to_c_string back to runtime/core #8296
• Include tensor shapes in get_broadcast_target_size error message #7944

[pytorch-bot]

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❌ 1 New Failure, 7 Pending

As of commit f98f69f with merge base 77f18b2 ():

NEW FAILURE - The following job has failed:

• trunk / test-coreml-delegate / macos-job (gh)
  The process '/opt/homebrew/bin/git' failed with exit code 128

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[swolchok]

this doesn't build because the new file is getting placed in portable_kernels__srcs by the buck2->cmake stuff, but kernels/portable/CMakeLists.txt doesn't respect portable_kernels__srcs and instead globs it itself! long-term plan is to stop using buck2 to drive CMake builds, so a short-term plan of "couple this with replacing dim_order_util's sizes_to_string" seems OK.