Fix check_nvlink_connections called with None group

[yurekami]

Summary

• Fixed a bug where check_nvlink_connections(group) was called before validating that group is not None
• This caused an AttributeError when users used the comm (mpi4py) parameter instead of group

Problem

# In Buffer.__init__:
check_nvlink_connections(group)  # Called here, but group can be None!

if group is not None:
    # ... handle group
elif comm is not None:
    # ... handle comm

When group=None and comm is provided, check_nvlink_connections would fail trying to call group.size().

Solution

Moved check_nvlink_connections(group) inside the if group is not None: block.

Changes

• deep_ep/buffer.py - Moved NVLink check after None validation

Test plan

• Create Buffer with group=None, comm=<mpi4py comm> and verify no AttributeError
• Create Buffer with valid group and verify NVLink check still runs

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