fix: Make VLLM model_impl configurable (#3358)

* Add ResponseFormat.DOCLANG and parsing branch in VLM pipeline

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* fix: Remove bogus preamble from VLM chat template

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* fix: Add include_stop_str_in_output in allowed VLLM sampling params

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* allow vllm model_impl to be defined

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* make VLLM model_impl default to auto

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

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Signed-off-by: Christoph Auer <cau@zurich.ibm.com>