added multithreading to OTPlanSampler for "exact" solver (#131)

* added multithreading to OTPlanSampler for "exact"
solver

* changed type hinting

torchcfm/optimal_transport.py

@@ -1,7 +1,7 @@
 import math
 import warnings
 from functools import partial
-from typing import Optional
+from typing import Optional, Union
 
 import numpy as np
 import ot as pot
@@ -18,6 +18,7 @@ def __init__(
         reg: float = 0.05,
         reg_m: float = 1.0,
         normalize_cost: bool = False,
+        num_threads: Union[int, str] = 1,
         warn: bool = True,
     ) -> None:
         """Initialize the OTPlanSampler class.
@@ -36,13 +37,16 @@ def __init__(
             normalizes the cost matrix so that the maximum cost is 1. Helps
             stabilize Sinkhorn-based solvers. Should not be used in the vast
             majority of cases.
+        num_threads: int or str, optional
+            number of threads to use for the "exact" OT solver. If "max", uses
+            the maximum number of threads.
         warn: bool, optional
             if True, raises a warning if the algorithm does not converge
         """
         # ot_fn should take (a, b, M) as arguments where a, b are marginals and
         # M is a cost matrix
         if method == "exact":
-            self.ot_fn = pot.emd
+            self.ot_fn = partial(pot.emd, numThreads=num_threads)
         elif method == "sinkhorn":
             self.ot_fn = partial(pot.sinkhorn, reg=reg)
         elif method == "unbalanced":