Fix used_mem_on_dev() units (GB instead of bytes)

[pdpino]

When calling ivy.used_mem_on_dev("cpu", process_specific=True), the output value is in bytes, though is expected to be in GB (per ivy docs). See psutil docs: psutil.Process(...).memory_info().rss is returned in bytes.

With this PR ivy.used_mem_on_dev(...) returns GB units for any argument provided