[NIXL][Metrics] Fix NIXL buckets

[markmc]

As per comments in vllm-project#26811

It's post times that need the smaller bucket size, not transfer duration.

Uniform 2kb to 16gb range:

>>> def human_size(bytes, units=[' bytes','KB','MB','GB','TB', 'PB', 'EB']):
...     """ Returns a human readable string representation of bytes """
...     return str(bytes) + units[0] if bytes < 1024 else human_size(bytes>>10, units[1:])
...
>>> [human_size(2**(10+i)) for i in range(1, 25, 2)]
['2KB', '8KB', '32KB', '128KB', '512KB', '2MB', '8MB', '32MB', '128MB', '512MB', '2GB', '8GB']

[NickLucche]

thank you!