Compile time scales with MPI ranks (and it shouldn't)

[mofeing]

cc @Todorbsc

we are seeing this strange behavior when running Reactant.compile in several MPI ranks at the same time. this should be flat, as there is no communication happening.

i have no idea of what's going around, except that MPI and Julia compiler don't like each other

Warning

note that this is not precompilation or load time, but just time taken by first call to Reactant.compile.

[mofeing]

The issue was unrelated to Reactant: MPI was initiating Julia in the same core. Thread pinning solved the problem.