openvino : update model conversion and README.md (#4003)

This commit updates the OpenVINO model conversion script and its
dependencies as they were currently not working.

The first issue was an import that needed updating for fix the following
error:
```console
  (openvino_conv_env) $ python convert-whisper-to-openvino.py --model base.en
  Traceback (most recent call last):
    File "/home/danbev/work/ai/whisper-work/models/convert-whisper-to-openvino.py", line 6, in <module>
      from openvino.runtime import serialize
  ModuleNotFoundError: No module named 'openvino.runtime'
```
And after that there was a missing dependency:
```console
ModuleNotFoundError: No module named 'onnxscript'
```

With the changes in this commit I was able to successfully convert the model
and run the inference using OpenVINO.

models/convert-whisper-to-openvino.py

@@ -3,7 +3,7 @@ import torch
 from whisper import load_model
 import os
 from openvino.frontend import FrontEndManager
-from openvino.runtime import serialize
+from openvino import serialize
 import shutil
 
 def convert_encoder(hparams, encoder, mname):

models/requirements-openvino.txt

@@ -1,2 +1,3 @@
 openvino>=2023.3.0
 openai-whisper
+onnxscript