[ML] Add pytorch_process rss memory stat to inference response

[darius-vil]

The idea is to:

1. Capture the stat in PytorchResultProcessor: https://github.com/elastic/elasticsearch/blob/01ace3927df065f1caf090653404f29688e8103a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/pytorch/process/PyTorchResultProcessor.java#L271
2. Make it available to /_ml/trained_model/_stats doc, which gets the stats from PytorchResultProcessor.getStats(): https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/pytorch/process/PyTorchResultProcessor.java#L231

[prodsecmachine]

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