Fix import of CAGrad and NashMTL in __init__.py

Author: ValerianRey

src/torchjd/aggregation/__init__.py

@@ -1,6 +1,5 @@
 from .aligned_mtl import AlignedMTL
 from .bases import Aggregator
-from .cagrad import CAGrad
 from .config import ConFIG
 from .constant import Constant
 from .dualproj import DualProj
@@ -9,9 +8,18 @@
 from .krum import Krum
 from .mean import Mean
 from .mgda import MGDA
-from .nash_mtl import NashMTL
 from .pcgrad import PCGrad
 from .random import Random
 from .sum import Sum
 from .trimmed_mean import TrimmedMean
 from .upgrad import UPGrad
+
+try:
+    from .cagrad import CAGrad
+except ImportError:  # The required dependencies are not installed
+    pass
+
+try:
+    from .nash_mtl import NashMTL
+except ImportError:  # The required dependencies are not installed
+    pass