Replace deprecated pynvml with nvidia-ml-py to fix FutureWarning

[Copilot]

The pynvml package has been deprecated in favor of nvidia-ml-py, causing a FutureWarning to be raised whenever batchflow.monitor is imported:

FutureWarning: The pynvml package is deprecated. Please install nvidia-ml-py instead. 
If you did not install pynvml directly, please report this to the maintainers of 
the package that installed pynvml for you.

This PR updates the dependency to use the maintained nvidia-ml-py package instead:

Changes made:

• Updated pyproject.toml to replace "pynvml" with "nvidia-ml-py" in monitor optional dependencies
• Updated error message in GPUResourceMonitor.__init__() to reference nvidia-ml-py

Why this works:
The nvidia-ml-py package provides the exact same pynvml module interface without deprecation warnings. No API changes were needed - all existing imports and function calls remain identical:

import pynvml  # Still works, but now from nvidia-ml-py
pynvml.nvmlInit()  # Same API
pynvml.nvmlDeviceGetHandleByIndex(0)  # Same API

Users installing batchflow[monitor] will now get the maintained package without any breaking changes to existing code.

Fixes #799.

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