5593 Raise error when GPU number is even greater than the dataset length (#5680)

Fixes #5593 .

### Description

This PR added error check in `DistributedSampler` for the case that GPU
number is greater than the dataset length and `unevenly divisible`.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: Nic Ma <nma@nvidia.com>

Author: Nic-Ma

monai/data/samplers.py

@@ -51,6 +51,8 @@ def __init__(
 
         if not even_divisible:
             data_len = len(dataset)  # type: ignore
+            if data_len < self.num_replicas:
+                raise ValueError("the dataset length is less than the number of participating ranks.")
             extra_size = self.total_size - data_len
             if self.rank + extra_size >= self.num_replicas:
                 self.num_samples -= 1

tests/test_sampler_dist.py

@@ -46,6 +46,12 @@ def test_uneven(self):
         if dist.get_rank() == 1:
             np.testing.assert_allclose(samples, np.array([2, 4]))
 
+    @DistCall(nnodes=1, nproc_per_node=2)
+    def test_uneven_less_data(self):
+        data = [1]
+        with self.assertRaises(ValueError):
+            DistributedSampler(dataset=data, shuffle=False, even_divisible=False)
+
     @DistCall(nnodes=1, nproc_per_node=2, timeout=120)
     def test_cachedataset(self):
         data = [1, 2, 3, 4, 5]