Add device-specific request validation in LLMEngine and modify request-specific asserts in TTModelRunner to not crash server instances

[skhorasganiTT]

Issue: #29

• Added _validate_device_inputs to LLMEngine for performing device-specific request validation (whereas the existing _validate_model_inputs and custom input processors can be used for device agnostic or model-specific request validation). Currently only supported for TT models through TTExecutor::validate_seq_group
• Changed sampling param assertions in TTModelRunner to ValueErrors which are triggered through the invocation of _validate_device_inputs mentioned above, so that bad requests return 400 errors instead of crashing the server
• Changed batch level assertions (during step execution) for same temp/top-p/top-k params to warnings to avoid server crashes (this cannot be handled through _validate_device_inputs since it has already reached step execution at which point any assertion or error will crash the server)
• Note: This PR also relies on [Llama3-70b] Separate vllm generator class and add prompt length validation in input processor tt-metal#15880

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