[WASI-NN] ggml: support grammar

[dm4]

• Support grammar for feat: ggml: support more parameters from llama.cpp #3314
• Example PR [Example] ggml: add grammar example second-state/WasmEdge-WASINN-examples#126

[juntao]

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Commit 1065aaccb5a8754648c90d5086995e48e454e415

This patch adds support for a grammar attribute in the Graph structure used by the WASI-NN GGML backend.

The following changes have been made:

1. A new conditional block has been introduced in the parseMetadata function at plugins/wasi_nn/ggml.cpp. This block checks if a "grammar" attribute is present in the JSON document. If this key is detected and can be retrieved as a string, the Graph's Grammar attribute is set to this value. If an error happens during the retrieval of this key, a log message is output and ErrNo::InvalidArgument is returned.

2. The setupGPTParam function has been modified to include the Graph's Grammar attribute into the GPTParams.sparams.grammar.

3. In the load function, a default value for GraphRef.Grammar is fetched from SamplingDefault.grammar.

4. The Graph structure at plugins/wasi_nn/ggml.h has been expanded with a std::string Grammar member.

Potential Problems:

1. An assumption is made that the "grammar" attribute will be a string. If there is a chance this assumption does not always hold, additional error handling is required.
2. No validation appears to be made for the actual contents of the "grammar" string. This might be an issue if certain grammar rules or structures are expected.
3. There's no null or empty string check for the "grammar" string; this might introduce errors or unexpected behavior if handled improperly later.
4. The parsing or loading of this new field has not been captured in any testing scenario. It would be beneficial to write tests to ensure this new field works as expected.
5. There's no context provided as to what the Grammar string represents, there should ideally be some comments explaining its purpose to improve code readability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.28%. Comparing base (0c65d8d) to head (3b76f0d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3318      +/-   ##
==========================================
- Coverage   77.29%   77.28%   -0.01%     
==========================================
  Files         185      185              
  Lines       25614    25614              
  Branches     5651     5651              
==========================================
- Hits        19799    19797       -2     
- Misses       4386     4388       +2     
  Partials     1429     1429              

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