Raise ValueError instead of a bare string in ParallelismConfig.get_device_mesh

[lollinng]

Problem

ParallelismConfig.get_device_mesh raises a bare string when device_type is missing:

raise ("You need to pass a device_type e.g cuda to build the device mesh")

In Python 3 you can only raise exception instances, so this throws
TypeError: exceptions must derive from BaseException instead of surfacing the
intended message — confusing for anyone who forgets to pass device_type.

Fix

Raise ValueError with the same message.

Testing done (CPU)

BEFORE  raise <str>        -> TypeError: exceptions must derive from BaseException
AFTER   raise ValueError   -> ValueError: You need to pass a device_type ...
REAL get_device_mesh(None) -> ValueError (good)

ruff clean.