Use Encoder as backend of AudioEncoder and VideoEncoder

[NicolasHug]

This PR replaces the AudioEncoder and VideoEncoder implemetations (both Python and C++) with that of the Encoder.
The existing tests of AudioEncoder and VideoEncoder are kept, so if they pass, it means the replacement is safe. We'll remove those tests in a follow-up PR.

Benchmarks show that there's no performance regression. These were obviously ran on main, not on this PR branch.

~/dev/torchcodec-cuda (main*) » python benchmarks/encoders/bench_multi_stream_vs_single_stream.py                                       nicolashug@nicolashug-fedora-PW0H326Y

==================================================
Duration: 3s
==================================================

--- Video encoding (cpu) ---
VideoEncoder:    med = 334.38ms +- 11.86
StreamingEncoder:med = 338.33ms +- 7.96

--- Video encoding (cuda) ---
VideoEncoder:    med = 265.74ms +- 1.37
StreamingEncoder:med = 265.88ms +- 1.36

--- Audio encoding (cpu) ---
AudioEncoder:    med = 0.87ms +- 0.27
StreamingEncoder:med = 0.74ms +- 0.04

==================================================
Duration: 10s
==================================================

--- Video encoding (cpu) ---
VideoEncoder:    med = 1042.70ms +- 20.33
StreamingEncoder:med = 1096.06ms +- 51.35

--- Video encoding (cuda) ---
VideoEncoder:    med = 265.92ms +- 2.52
StreamingEncoder:med = 265.72ms +- 1.21

--- Audio encoding (cpu) ---
AudioEncoder:    med = 1.98ms +- 0.31
StreamingEncoder:med = 2.17ms +- 0.16

[pytorch-bot]

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