Let getFramesAtIndices rely on getFrameAtIndex

[NicolasHug]

Small refactor discussed in #260

[ahmadsharif1]

Can you run the benchmark just to be paranoid?

[NicolasHug]

On main:

swscale
med = 21.88ms +- 2.99
filtergraph
med = 25.42ms +- 5.84

This PR:

swscale
med = 21.92ms +- 1.66
filtergraph
med = 25.83ms +- 4.60

import torch
from time import perf_counter_ns

from torchcodec.decoders._core import (
    _add_video_stream,
    create_from_file,
    get_frames_at_indices,
    scan_all_streams_to_update_metadata,
)

import torch
from time import perf_counter_ns


def bench(f, color_conversion_library, num_exp=100):

    times = []
    for _ in range(num_exp):
        VIDEO_PATH = "./test/resources/nasa_13013.mp4"
        decoder = create_from_file(VIDEO_PATH)
        _add_video_stream(
            decoder,
            color_conversion_library=color_conversion_library,
        )
        scan_all_streams_to_update_metadata(decoder)

        start = perf_counter_ns()
        f(decoder)
        end = perf_counter_ns()
        times.append(end - start)
    return torch.tensor(times).float()

def report_stats(times, unit="ms"):
    mul = {
        "ns": 1,
        "µs": 1e-3,
        "ms": 1e-6,
        "s": 1e-9,
    }[unit]
    times = times * mul
    std = times.std().item()
    med = times.median().item()
    print(f"{med = :.2f}{unit} +- {std:.2f}")
    return med



NUM_EXP = 100
stream_index=3
frame_indices = list(range(100))

def _get_frames_at_indices(decoder):
    get_frames_at_indices(decoder=decoder, stream_index=stream_index, frame_indices=frame_indices)

for color_conversion_library in ("swscale", "filtergraph"):
    print(color_conversion_library)
    times = bench(_get_frames_at_indices, color_conversion_library=color_conversion_library, num_exp=NUM_EXP)
    report_stats(times)