py : new conversion script (#545)

Current status: Working, except for the latest GPTQ-for-LLaMa format
  that includes `g_idx`.  This turns out to require changes to GGML, so
  for now it only works if you use the `--outtype` option to dequantize it
  back to f16 (which is pointless except for debugging).

  I also included some cleanup for the C++ code.

  This script is meant to replace all the existing conversion scripts
  (including the ones that convert from older GGML formats), while also
  adding support for some new formats.  Specifically, I've tested with:

  - [x] `LLaMA` (original)
  - [x] `llama-65b-4bit`
  - [x] `alpaca-native`
  - [x] `alpaca-native-4bit`
  - [x] LLaMA converted to 'transformers' format using
        `convert_llama_weights_to_hf.py`
  - [x] `alpaca-native` quantized with `--true-sequential --act-order
        --groupsize 128` (dequantized only)
  - [x] same as above plus `--save_safetensors`
  - [x] GPT4All
  - [x] stock unversioned ggml
  - [x] ggmh

  There's enough overlap in the logic needed to handle these different
  cases that it seemed best to move to a single script.

  I haven't tried this with Alpaca-LoRA because I don't know where to find
  it.

  Useful features:

  - Uses multiple threads for a speedup in some cases (though the Python
    GIL limits the gain, and sometimes it's disk-bound anyway).

  - Combines split models into a single file (both the intra-tensor split
    of the original and the inter-tensor split of 'transformers' format
    files).  Single files are more convenient to work with and more
    friendly to future changes to use memory mapping on the C++ side.  To
    accomplish this without increasing memory requirements, it has some
    custom loading code which avoids loading whole input files into memory
    at once.

  - Because of the custom loading code, it no longer depends in PyTorch,
    which might make installing dependencies slightly easier or faster...
    although it still depends on NumPy and sentencepiece, so I don't know
    if there's any meaningful difference.  In any case, I also added a
    requirements.txt file to lock the dependency versions in case of any
    future breaking changes.

  - Type annotations checked with mypy.

  - Some attempts to be extra user-friendly:

      - The script tries to be forgiving with arguments, e.g. you can
        specify either the model file itself or the directory containing
        it.

      - The script doesn't depend on config.json / params.json, just in
        case the user downloaded files individually and doesn't have those
        handy.  But you still need tokenizer.model and, for Alpaca,
        added_tokens.json.

      - The script tries to give a helpful error message if
        added_tokens.json is missing.

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