Using numpy.array to store string object is not memory efficient.

https://github.com/awslabs/graphstorm/blob/main/python/graphstorm/gconstruct/file_io.py#L153
The default behavior of numpy.array(string) will allocate too much memory than required, which is not efficient.

Start by reading python/graphstorm/gconstruct/file_io.py at line 153 and inspect how numpy.array stores the string object. Compare the current allocation with a more memory-efficient representation, then verify that string handling remains correct and that the reported excess memory is reduced.

Written by the indexing model from the issue text.