fix: use new ruamel yaml APIs

[azhou-determined]

Ticket

Description

ruamel changed their APIs and previous method call no longer exists.

Test Plan

run MNIST locally: go to examples/tutorials/mnist_pytorch and run python3 train.py, make sure it runs and exits successfully.

Checklist

• Changes have been manually QA'd
• New features have been approved by the corresponding PM
• User-facing API changes have the "User-facing API Change" label
• Release notes have been added as a separate file under docs/release-notes/
  See Release Note for details.
• Licenses have been included for new code which was copied and/or modified from any external code

[netlify]

✅ Deploy Preview for determined-ui canceled.

Name	Link
🔨 Latest commit	7bd16a7
🔍 Latest deploy log	https://app.netlify.com/sites/determined-ui/deploys/66d0b7974a6fec000878b4d2

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.23%. Comparing base (a605f00) to head (7bd16a7).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #9882       +/-   ##
===========================================
- Coverage   54.70%   44.23%   -10.48%     
===========================================
  Files        1261      658      -603     
  Lines      156681    67002    -89679     
  Branches     3596        0     -3596     
===========================================
- Hits        85710    29636    -56074     
+ Misses      70840    37366    -33474     
+ Partials      131        0      -131     

Flag	Coverage Δ
harness	40.34% <ø> (-32.29%)	⬇️
web	?

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see 693 files with indirect coverage changes