Manage runtime environments (#452)

* Add `Env` class that manages all runtime environments.
* Changed `NPKIT_DUMP_DIR` to `MSCCLPP_NPKIT_DUMP_DIR`.

python/test/executor_test.py

@@ -8,6 +8,7 @@ from mscclpp import (
     ExecutionPlan,
     PacketType,
     npkit,
+    env,
 )
 import mscclpp.comm as mscclpp_comm
 from mscclpp.utils import KernelBuilder, GpuBuffer, pack
@@ -171,8 +172,8 @@ def main(
     mscclpp_group = mscclpp_comm.CommGroup(MPI.COMM_WORLD)
     cp.cuda.Device(mscclpp_group.my_rank % mscclpp_group.nranks_per_node).use()
     executor = Executor(mscclpp_group.communicator)
-    npkit_dump_dir = os.getenv("NPKIT_DUMP_DIR")
-    if npkit_dump_dir is not None:
+    npkit_dump_dir = env().npkit_dump_dir
+    if npkit_dump_dir != "":
         npkit.init(mscclpp_group.my_rank)
     execution_plan = ExecutionPlan(execution_plan_path)
     collective = execution_plan.collective()