build: add torch to requirements.txt

This commit adds torch to requirements.txt. This is needed to run the
convert-hf-to-gguf.py.

The motivation for this is that currently running convert-hf-to-gguf.py
will produce the following error:
```console
$ python3 -m venv venv
$ source venv/bin/activate
(venv) $ pip install -r requirements.txt
Collecting numpy==1.24.4
Collecting sentencepiece==0.1.98
Collecting gguf>=0.1.0
Installing collected packages: sentencepiece, numpy, gguf
Successfully installed gguf-0.5.1 numpy-1.24.4 sentencepiece-0.1.98

(venv) $ python convert-hf-to-gguf.py --help
Traceback (most recent call last):
  File "llama.cpp/convert-hf-to-gguf.py", line 16, in <module>
    import torch
ModuleNotFoundError: No module named 'torch'
```
With this commit the script can be run and shows the help output.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

Author: danbev

requirements.txt

@@ -1,3 +1,4 @@
 numpy==1.24.4
 sentencepiece==0.1.98
 gguf>=0.1.0
+torch==2.1.1