[VLM][BugFix] Make sure that multi_modal_kwargs can broadcast properly with ring buffer.

[xwjiang2010]

The premerge test for #5880 didn't incorporate the latest code in distributed backend. The postmerge test caught this bug.

Additionally updated type hinting for the key of tensor_dict to be specifically str.

Example capture:

~~~~~~~~~~~~broadcasting tensor_dict.keys() = dict_keys(['input_tokens', 'input_positions', 'selected_token_indices', 'lora_requests', 'lora_mapping', 'multi_modal_kwargs', 'num_prefill_tokens', 'num_decode_tokens', 'slot_mapping', 'num_prefills', 'seq_lens', 'seq_lens_tensor', 'max_query_len', 'max_prefill_seq_len', 'max_decode_seq_len', 'query_start_loc', 'seq_start_loc', 'context_lens_tensor', 'block_tables', 'use_cuda_graph', '_cached_prefill_metadata', '_cached_decode_metadata'])
(VllmWorkerProcess pid=86768) ~~~~~~~~~~~~recving tensor_dict.keys() = dict_keys(['input_tokens', 'input_positions', 'selected_token_indices', 'lora_requests', 'lora_mapping', 'multi_modal_kwargs', 'num_prefill_tokens', 'num_decode_tokens', 'slot_mapping', 'num_prefills', 'seq_lens', 'seq_lens_tensor', 'max_query_len', 'max_prefill_seq_len', 'max_decode_seq_len', 'query_start_loc', 'seq_start_loc', 'context_lens_tensor', 'block_tables', 'use_cuda_graph', '_cached_prefill_metadata', '_cached_decode_metadata'])

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