Timeline: Add MEMCPY_IN_FUSION_BUFFER event for Reducescatter

[maxhgerlach]

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

The MEMCPY_IN_FUSION_BUFFER event accidentally wasn't recorded for NCCLReducescatter, which made Horovod timelines a bit confusing to read.

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.

[maxhgerlach]

Build failures with macOS are unrelated to this change (#3809).

[github-actions]

Unit Test Results

  1 155 files  ±0    1 155 suites  ±0   13h 21m 10s ⏱️ + 43m 24s
     841 tests ±0       788 ✔️ ±0       53 💤 ±0  0 ❌ ±0 
23 809 runs  ±0  17 023 ✔️ ±0  6 786 💤 ±0  0 ❌ ±0 

Results for commit 0d6d6ff. ± Comparison against base commit 4ad1d43.

[github-actions]

Unit Test Results (with flaky tests)

  1 220 files  +  39    1 220 suites  +39   13h 44m 51s ⏱️ + 57m 38s
     841 tests ±    0       787 ✔️ ±    0       53 💤 ±  0  1 ❌ ±0 
24 654 runs  +507  17 828 ✔️ +483  6 821 💤 +21  5 ❌ +3 

For more details on these failures, see this check.

Results for commit 0d6d6ff. ± Comparison against base commit 4ad1d43.

[romerojosh]

LGTM!