Running htex/exex with TorqueProvider hardcodes ppn=1

[annawoodard]

This is related to #557 and #660.

We set it here. I'm not sure how to get around this without asking for the number of cores/node ahead of time. Maybe there is some PBS magic we can do to avoid setting it at all or set it to # of cores?

[annawoodard]

Note that I think the main difficulty here stems from the fact that we use one parameter (tasks_per_node) to describe two things: 1) the number of loops the launcher executes and 2) the number of processes we plan to run on the node. I don't see an alternative except to add a second parameter to provider.submit. As a quick hack for the exex case, we can just set tasks_per_node=ranks_per_node-- this breaks the launcher, but that won't affect most configs, as we're currently recommending you usually use the SimpleLauncher with exex anyways.

[annawoodard]

Note this issue is critical to have exex work out-of-the-box on BW.

[benclifford]

crossref #3616