[WIP] Upstream encoder/decoder support based on multiple blocktables

[afeldman-nm]

vLLM currently supports decoder-only models. This PR

• Adds support for T5 (encoder/decoder model) using the latest vLLM Attention wrapper
• Augments SequenceGroups with optional encoder sequence representations
• Modifies block_manager to support both self- and cross-attention blocktables
• model_runner prepare_prompt and prepare_decode can optionally prepare input_metadata for encoder-self-attention and decoder-cross-attention, in addition to decoder-self-attention (current supported behavior)

This PR when finished

• Addresses vLLM issue 187 Adding support for encoder-decoder models, like T5 or BART vllm-project/vllm#187
• Represents the completion of work started by js8544 to integrate encoder/decoder support Add Encoder-decoder model support and T5 Model support vllm-project/vllm#3117
• Supports work on Whisper integration [WiP] Whisper Implementation #147 Whisper support vllm-project/vllm#180

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[andy-neuma]

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