Disallow timing calls in leaf tasks

[manopapad]

Bug report due to @elliottslaughter

Timing calls ends up doing a Legion runtime call, that are presumably not allowed in leaf tasks. E.g. the following program will hang:

import cupy as np
from legate.core.task import task, InputArray, OutputArray
from legate.core import get_machine, TaskTarget, VariantCode
import cupynumeric as cpn
from legate.timing import time

@task(variants=(VariantCode.GPU,))
def foo_in_out(in_store: InputArray, out_store: OutputArray, start_time: int) -> None:
    in_store = np.asarray(in_store)
    out_store = np.asarray(out_store)
    out_store[:] = in_store[:]
    print((time() - start_time)/1e6)

with get_machine().only(TaskTarget.GPU):
    start_time = time()
    in_store = cpn.array([1, 2, 3])
    out_store = cpn.array([1, 2, 3])
    foo_in_out(in_store, out_store, int(start_time))
    print(out_store)

We should not allow timing calls (or most runtime calls for that mater) in leaf tasks.

[manopapad]

Actually @lightsighter will do the right thing at the Legion level, i.e. emit a timing op if on a non-leaf task, and just do the timing eagerly if in a leaf task.

[lightsighter]

https://gitlab.com/StanfordLegion/legion/-/merge_requests/1633

[manopapad]

Note to self: before we can close this, we also need to make sure the timing operation is launched using the appropriate context for the current task.

[lightsighter]

Yes, that is still required otherwise bad things could happen (will be checked for you in what is probably the next Legion release).

[lightsighter]

Merged.