[FEATURE] Consolidate 6+ related use/disable: bool args in from_quantized into single backend: Backend

[Qubitium]

The following args will be merged into single backed: Backend = Backend.AUTO

• use_triton: bool,
• disable_exllama: bool = False,
• disable_exllamav2: bool = False,
• use_marlin: bool = False,
• use_bitblas: bool = True,

Reason: It is not only super confusing for users to use correctly (matrix condition of passive binary toggles), even project developers ran into multiple bugs due to these passive switches. We can't keep adding more binary toggles every time we add a backend/kernel/runtime. Becoming unmaintainable and unusable by both end-users and project devs.

Prelim design:

class Backend(ENUM):
   AUTO # choose the fastest one based on quant model compatibility
   CUDA_OLD
   CUDA 
   TRITON_V2
   EXLLAMA
   EXLLAMA_V2
   MARLIN
   BITBLAS

This is the final todo for 0.9.1 Release.