benchdnn: graph: check partition number by default

[TaoLv]

Currently on main branch, --expected-n-partitions is 0 by default (if not provided in benchdnn command line), which means to skip checking the partition number returned from an input graph.
This PR changes the default value of --expected-n-partitions to 1 which means the input graph should be fused into one partition.
Most of the cases we have in CI/Nightly should fused into one partition. With this check enabled by default, we can detect that if a PR breaks a fusion into multiple partitions. Before this change, those multiple partitions will silently pass the correctness check of benchdnn.
Apparently, we still have a few cases which cannot fuse into one partition due to:

• the cases previously were added for graph compiler backend.
• the cases can fuse on cpu but not on gpu, or vice versa.

Partition number check is skipped for these cases explicitly by setting --expected-n-partitions=0 in the command lines. We can track and remove these settings when the fusions are implemented in the future.

[TaoLv]

make test
enable benchdnn_nightly
disable benchdnn_all
enable benchdnn_graph