pass *args, **kwargs to Optimizer.zero_grad

[njzjz]

Checklist before submitting

• Did you read the contributor guide?
• Did you update the docs?
• Did you write any tests to validate this change?
• Did you update the CHANGELOG, if this change affects users?

Description

PyTorch 1.7 added set_to_none to Optimizer.zero_grad (see pytorch/pytorch@c515881). Horovod needs to be compatible with PyTorch 1.5, so I pass *args, **kwargs to zero_grad in this PR, which is safe in both old and new PyTorch versions.

Review process to land

1. All tests and other checks must succeed.
2. At least one member of the technical steering committee must review and approve.
3. If any member of the technical steering committee requests changes, they must be addressed.

[github-actions]

Unit Test Results

   568 files   -   156    568 suites   - 156   6h 43m 3s ⏱️ - 1h 23m 4s
   887 tests ±    0    692 ✅  -    76    195 💤 + 76  0 ❌ ±0 
12 738 runs   - 3 471  8 641 ✅  - 2 712  4 097 💤  - 759  0 ❌ ±0 

Results for commit 8fb5874. ± Comparison against base commit 9f88e1d.

This pull request skips 76 tests.

test.parallel.test_mxnet1.MX1Tests ‑ test_gluon_trainer
test.parallel.test_mxnet1.MX1Tests ‑ test_gpu_required
test.parallel.test_mxnet1.MX1Tests ‑ test_horovod_allreduce_cpu_gpu_error
test.parallel.test_mxnet1.MX1Tests ‑ test_horovod_grouped_allgather_cpu_gpu_error
test.parallel.test_mxnet1.MX1Tests ‑ test_horovod_grouped_allreduce_cpu_gpu_error
test.parallel.test_tensorflow.TensorFlowTests ‑ test_gpu_required
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_fused_gpu
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_gpu
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_grad_gpu
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_variable_size_fused_gpu
…

[github-actions]

Unit Test Results (with flaky tests)

   568 files   -   320    568 suites   - 320   6h 43m 3s ⏱️ - 2h 15m 38s
   887 tests ±    0    692 ✅  -    76    195 💤 +   76  0 ❌ ±0 
12 738 runs   - 7 501  8 641 ✅  - 5 148  4 097 💤  - 2 353  0 ❌ ±0 

Results for commit 8fb5874. ± Comparison against base commit 9f88e1d.

This pull request skips 76 tests.

test.parallel.test_mxnet1.MX1Tests ‑ test_gluon_trainer
test.parallel.test_mxnet1.MX1Tests ‑ test_gpu_required
test.parallel.test_mxnet1.MX1Tests ‑ test_horovod_allreduce_cpu_gpu_error
test.parallel.test_mxnet1.MX1Tests ‑ test_horovod_grouped_allgather_cpu_gpu_error
test.parallel.test_mxnet1.MX1Tests ‑ test_horovod_grouped_allreduce_cpu_gpu_error
test.parallel.test_tensorflow.TensorFlowTests ‑ test_gpu_required
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_fused_gpu
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_gpu
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_grad_gpu
test.parallel.test_tensorflow.TensorFlowTests ‑ test_horovod_allgather_variable_size_fused_gpu
…