fix: handle empty DC-AE temporal sampling configs

[primorLee]

Summary

• Treat empty EncoderConfig.temporal_downsample and DecoderConfig.temporal_upsample sequences as disabling temporal resampling.
• Add CPU-local regressions for both default video encoder and decoder configurations.

Problem

Both temporal configuration fields default to empty tuples. The encoder and decoder compared those tuples to an empty list, so the conditions were true and construction indexed an empty tuple at the first resampling stage. The original change fixed the decoder path; the follow-up audit found and fixed the identical encoder path.

Validation

• Focused CPU test with a local optional-dependency import bootstrap: tests/models/dc_ae/test_decoder.py — 2 passed
• ruff check opensora/models/dc_ae/models/dc_ae.py tests/models/dc_ae/test_decoder.py
• git diff --check