[DGL] Allow passing in xxx_nodes.txt, xxx_edges.txt, xxx_stats.txt files by path to dglpart.c, rather than hard-coding the names

[erickim555]

Currently, dglpart.c assumes the following about the inputs:

1. The nodes, edges, stats files are named "{STEM}_nodes.txt", "{STEM}_edges.txt", "{STEM}_stats.txt", where {STEM} is a common prefix
2. These nodes/edges/stats files all reside at the working dir where the cmd is invoked

This is a little inflexible. It'd be nice if we instead could invoke pm_dglpart by passing in these paths explicitly, eg something like:

# "some_graph_name" is also explicitly passed in as the graph name
pm_dglpart \
path/to/my_graph/nodes/my_nodes.txt \
path/to/my_graph/edges/edges.txt \
path/to/stats/stats.txt \
some_graph_name \
2

Perhaps this would break downstream consumers that assume that the graph name is encoded in the output files, eg p0-xxx-nodes.txt. So, maybe leaving this as-is is a reasonable compromise to maintain current behavior...