Spark/Lightning: add missing tranform_spec for Petastorm datamodule

[chongxiaoc]

Signed-off-by: Chongxiao Cao chongxiaoc@uber.com

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 #3540 .

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

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

Results for commit d899497. ± Comparison against base commit 464c82e.

[github-actions]

Unit Test Results (with flaky tests)

     905 files  ±0       905 suites  ±0   9h 45m 56s ⏱️ + 9m 25s
     768 tests ±0       725 ✔️ ±0       43 💤 ±0  0 ❌ ±0 
21 164 runs  ±0  14 999 ✔️ ±0  6 165 💤 ±0  0 ❌ ±0 

Results for commit d899497. ± Comparison against base commit 464c82e.

[Tixxx]

LGTM