[v0.11.0][Fix] Prevent memory leak in MLA decode graph (#3743) (#3774)

### What this PR does / why we need it?
The cache for MLA decode graph parameters was holding strong references
to tensors, preventing them from being garbage collected and leading to
increased memory usage.

This change wraps the cached tensors in weak references, allowing them
to be deallocated when no longer in use and reducing overall memory
pressure.

### Does this PR introduce _any_ user-facing change?
None.

### How was this patch tested?
None.

Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>

vllm_ascend/compilation/acl_graph.py

@@ -212,7 +212,6 @@ def update_attn_params(update_stream, forward_context, runtime_shape):
             seq_lens,
             output,
         ) = param
-        # block_table = forward_context.attn_metadata[key].block_tables
         seq_lens = forward_context.attn_metadata[key].seq_lens
         torch_npu_check = version_check()
 
@@ -258,8 +257,7 @@ def update_mla_attn_params(update_stream, forward_context, runtime_shape,
     ):
         (q_nope, k_nope, q_pe, k_pe, num_heads, num_kv_heads, input_layout,
          spec_attn_mask, sparse_mode, scale, block_table, block_size,
-         seq_lens_list, actual_seq_lengths, workspace, attn_output,
-         softmax_lse) = param
+         seq_lens_list, actual_seq_lengths, attn_output, softmax_lse) = param
         seq_lens_list = forward_context.attn_metadata[key].decode.seq_lens_list
         if speculative_config and speculative_config.method == "deepseek_mtp":
             actual_seq_lengths = forward_context.attn_metadata[
@@ -295,7 +293,7 @@ def update_mla_attn_params(update_stream, forward_context, runtime_shape,
                 block_size=block_size,
                 actual_seq_lengths_kv=seq_lens_list,
                 actual_seq_lengths=actual_seq_lengths,
-                workspace=workspace,
+                workspace=graph_params.workspaces.get(runtime_shape),
                 out=[attn_output, softmax_lse])
             torch.npu.graph_task_update_end(update_stream)
 
@@ -329,7 +327,7 @@ def set_graph_params(aclgraph_capture_sizes: set[int]):
     )
 
 
-def update_graph_params_workspaces(num_tokens: int, workspace: int):
+def update_graph_params_workspaces(num_tokens: int, workspace: Any):
     global _graph_params
     if _graph_params is not None:
         _graph_params.workspaces[num_tokens] = workspace