Pass packets as const AVPacket& through the decode seam

[NicolasHug]

DeviceInterface::send_packet() only ever reads the packet - the default
implementation hands it to avcodec_send_packet(), which takes a const one.
Except that BetaCuda's bitstream filter path did not: av_bsf_send_packet()
takes ownership of what it is given and resets it, so sending a packet
through the seam could empty it, silently, with nothing in the signature
saying so.

SingleStreamDecoder never noticed, because its packet is dead after the
send. PacketDecoder did: the packet it is handed is a user-visible Python
object that has to survive being decoded, so it referenced it into a
throwaway packet before every single send - a per-packet allocation, on
every device, to defend against a contract that was never stated.

State the contract instead. The seam takes a const AVPacket&, and the one
implementation that needs ownership references the packet itself, where the
reason for it is local.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/meta-pytorch/torchcodec/1691

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 3 Unrelated Failures

As of commit fe3317e with merge base 6f705bb ():

NEW FAILURE - The following job has failed:

• Linux CUDA x86 / install-and-test (3.10, 13.0, 4) (gh)
  test/test_decoders.py::TestBlocks::test_cpu_fallback_upload_keeps_full_range[422]

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Linux CUDA x86 / install-and-test (3.10, 13.0, 6) (gh) (detected as infra flaky with no log or failing log classifier)
• Linux CUDA x86 / install-and-test (3.10, 13.2, 4) (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Linux CUDA x86 / install-and-test (3.10, 13.2, 6) (gh) (trunk failure)

This comment was automatically generated by Dr. CI and updates every 15 minutes.