Consider "tool_calls" instead of "content" in messages

[grf53]

In the latest document of transformers(link), a message containing 'tool calls' contains member with key "tool_calls" instead of "content".

{
  "role": "assistant",
  "tool_calls": [
    {
      "type": "function",
      "function": {
        "name": "multiply",
        "arguments": {
          "a": 5,
          "b": 6
        }
      }
    }
  ]
}

The 'tool call' messages are not necessarily included as input messages because they are model outputs rather than model inputs. However, they are necessary to provide context during the process of providing tool execution results or after.

Currently, a message without "content" cannot be put into chat_template::apply() due to the validation logic as follows.

minja/include/minja/chat-template.hpp

Lines 398 to 399 in fcb5a0d

	if (!message.contains("role") || !message.contains("content")) {
	throw std::runtime_error("message must have 'role' and 'content' fields: " + message.dump());

When referencing other lines of code, it seems like it's required to include a member with a "content" key, even if it's null or an empty string. However, I don't think it's very persuasive to require even an empty "content" when "tool_calls" can replace it. So I propose to require only one of "content" and "tool_calls", as in this PR.

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[grf53]

@ochafik Hello, I think you've seen this. How is your opinion on this changes? If there are more points to modify, please let me know. :)