fix(train): pad training batches with the model's pad id - #358

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065, astral-sh/setup-uv@d0d8abe699bfb85fec6de9f7adb5ae17292296ff, codecov/codecov-action@b9fd7d16f6d7d1b5d2bec1a2887e65ceed900238. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/