Models like Gemma 2 and its fine-tunes reject the system role in their
chat template. generate() always prepends a system message, so the
template raises 'System role not supported' and the dtype loop gives up,
making these models impossible to load.
Detect once (right after the tokenizer loads) whether the template
accepts a system role, by rendering a probe chat and checking a sentinel
system message survives in the output (this catches both templates that
raise and templates that silently drop the role). When the role isn't
supported, fold every system message into the following user message
(dropping empty system prompts), so a system role anywhere in the chat
is removed before rendering, not just a leading one. Templates that
support the system role are unaffected, so their rendered chats and
resulting weights are unchanged.
Supersedes #483.
Co-authored-by: jayhemnani9910 <jayhemnani9910@users.noreply.github.com>
