Support Shar export of multi-channel, multi-source recording and cuts with start>0

[pzelasko]

@desh2608 can you please try this out before we merge? The tests are working but I'd rather double-check.

[desh2608]

Sorry, I found that there is some issue when using >1 jobs for writing:

Shard progress: 0it [00:04, ?it/s]
concurrent.futures.process._RemoteTraceback:
"""
Traceback (most recent call last):
  File "/home/hltcoe/draj/.conda/envs/torch2/lib/python3.8/concurrent/futures/process.py", line 239, in _process_worker
    r = call_item.fn(*call_item.args, **call_item.kwargs)
  File "/exp/draj/mini_scale_2022/lhotse/lhotse/cut/set.py", line 3405, in _export_to_shar_single
    for cut in cuts:
  File "/exp/draj/mini_scale_2022/lhotse/lhotse/lazy.py", line 165, in values
    yield from self
  File "/exp/draj/mini_scale_2022/lhotse/lhotse/lazy.py", line 216, in __iter__
    yield from map(deserialize_item, self.source)
  File "/exp/draj/mini_scale_2022/lhotse/lhotse/lazy.py", line 186, in __iter__
    for line in f:
  File "/home/hltcoe/draj/.conda/envs/torch2/lib/python3.8/gzip.py", line 305, in read1
    return self._buffer.read1(size)
  File "/home/hltcoe/draj/.conda/envs/torch2/lib/python3.8/_compression.py", line 68, in readinto
    data = self.read(len(byte_view))
  File "/home/hltcoe/draj/.conda/envs/torch2/lib/python3.8/gzip.py", line 498, in read
    raise EOFError("Compressed file ended before the "
EOFError: Compressed file ended before the end-of-stream marker was reached
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "local/prepare_chime6_ihm.py", line 83, in <module>
    prepare_chime6_ihm()
  File "local/prepare_chime6_ihm.py", line 69, in prepare_chime6_ihm
    shards = cuts.to_shar(
  File "/exp/draj/mini_scale_2022/lhotse/lhotse/cut/set.py", line 618, in to_shar
    partial_paths = f.result()
  File "/home/hltcoe/draj/.conda/envs/torch2/lib/python3.8/concurrent/futures/_base.py", line 437, in result
    return self.__get_result()
  File "/home/hltcoe/draj/.conda/envs/torch2/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
EOFError: Compressed file ended before the end-of-stream marker was reached

[pzelasko]

OK, we'll resolve it in a follow up PR