What does this PR do?

Adds a HealthChecker callback to monitor node health. Will log a warning if GPUs on a node appear to be in poor health. Provides an optional interface to send a message via Slack webhook.

Note: this adds the pynvml and slack_sdk to the dependencies. Unsure if this should be in the base install or a separate tag.

TODO:

• Run some LLM 1B training runs to verify minimal throughput impact

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