regression test (#234)

Summary:
- add a test that uses fixtures to validate against previous runs of the test
- the fixutres can be written using `WRITE_FIXTURE=true pytest -vs ./torchft/test_diloco_mocked_updates.py`
- when writing fixtures, the test also numerically validates the implementation of streaming diloco

Author: tushar00jain

test_fixtures/torchft.diloco_regression_test.DiLoCoMockedUpdateTest.test_diloco_mocked_updates_0.json

@@ -18,6 +18,7 @@
 from torch.distributed.pipelining import SplitPoint, pipeline
 from torch.distributed.tensor import DTensor, Replicate
 
+from torchft._test.diloco_trainer import DiLoCoTrainer, MultiMyModel
 from torchft._torchft import LighthouseServer
 from torchft.device_mesh import ft_init_device_mesh
 from torchft.local_sgd import DiLoCo, LocalSGD
@@ -33,7 +34,6 @@
     ProcessGroupBabyNCCL,
     ProcessGroupGloo,
 )
-from torchft.test.diloco_trainer import DiLoCoTrainer, MultiMyModel
 
 logger: logging.Logger = logging.getLogger(__name__)