Make HorovodVersionMismatchError subclass ImportError

[amogkam]

HorovodVersionMismatchError is raised during import horovod.torch when the current Pytorch version does not match with the torch version during Horovod installation time. Therefore, this custom error should subclass the built-in Python ImportError so that it is caught by except ImportError statements in upstream code.

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

Fixes # (issue).

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.

[chongxiaoc]

LGTM

[github-actions]

Unit Test Results

     821 files  ±0       821 suites  ±0   9h 16m 32s ⏱️ - 25m 53s
     768 tests ±0       725 ✔️ ±0       43 💤 ±0  0 ❌ ±0 
18 950 runs  ±0  13 655 ✔️ ±0  5 295 💤 ±0  0 ❌ ±0 

Results for commit 2b173cd. ± Comparison against base commit 7707267.

[github-actions]

Unit Test Results (with flaky tests)

     905 files   -   37       905 suites   - 37   9h 39m 14s ⏱️ - 35m 1s
     768 tests ±    0       725 ✔️ +    1       43 💤 ±    0  0 ❌  - 1 
21 164 runs   - 884  14 999 ✔️  - 646  6 165 💤  - 237  0 ❌  - 1 

Results for commit 2b173cd. ± Comparison against base commit 7707267.