add environment variable "OMP_NUM_THREADS"

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add environment variable "OMP_NUM_THREADS" (default value :1) in CPU instances, and write this value into TorchServe config.properties.

**How would this feature be used? Please describe.**
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[Here](https://github.com/pytorch/serve/issues/1642) is a related ticket in TorchServe side.

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**Additional context**
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Locate the CPU-instance configuration entry point and the code that writes TorchServe config.properties. Read the related TorchServe issue #1642 for expected behavior, then verify that OMP_NUM_THREADS defaults to 1 for CPU instances and that the selected value is written to config.properties.

Written by the indexing model from the issue text.