[CLI] Report KV cache memory usage in mlc_llm compile

[CharlieFRuan]

This PR prints out the memory usage of KV cache: MB for one token's KV cache, and the total MB for model weights + intermediate buffers + a 4K-long KV cache.

If somehow the required fields are not present in config and metadata (e.g. for an old model), we do nothing.

Sample output in CLI:

[2025-05-03 21:44:24] INFO model_metadata.py:94: Total memory usage without KV cache: 2254.16 MB (Parameters: 923.16 MB. Temporary buffer: 1331.00 MB)
[2025-05-03 21:44:24] INFO model_metadata.py:128: KV cache size: 0.11 MB per token in the context window
[2025-05-03 21:44:24] INFO model_metadata.py:133: Total memory usage with a 4K KV cache: 2702.16 MB