mpi/pmix: Assume that all nodes have same arch by default

This helps to slightly improve the performance:
size    UCXv4   UCXv5
1       20.0    20.3
2       20.0    18.3
4       20.0    17.7
8       21.0    20.3
16      21.0    20.3
32      21.0    20.3
64      22.0    21.0
128     22.0    21.0
256     22.7    21.7
512     23.0    22.0
1024    23.0    22.0
2048    24.7    23.0
4096    27.3    26.0
8192    30.7    29.0
16384   33.0    31.7
32768   38.7    37.3
65536   51.0    49.0
131072  76.0    74.3
262144  2275.0  2227.3
524288  2969.7  2801.0
1048576 3835.3  3795.0
2097152 6274.7  6292.3
4194304 10728.0 10314.3

Signed-off-by: Artem Polyakov <artpol84@gmail.com>