[DSV3.2][MTP] Optimize Indexer MTP handling (#36723)

Signed-off-by: Benjamin Chislett <bchislett@nvidia.com>

vllm/v1/attention/backends/mla/indexer.py

@@ -384,12 +384,14 @@ class DeepseekV32IndexerMetadataBuilder(AttentionMetadataBuilder):
 
                 # [7, 6, 8, 0] -> [7, 7, 7, 6, 8, 8, 8, 8]
                 expanded_base = torch.repeat_interleave(
-                    seq_lens - decode_lens, decode_lens
+                    seq_lens - decode_lens, decode_lens, output_size=actual_expanded
                 )
 
                 # [0, 3, 4, 8] -> [0, 0, 0, 3, 4, 4, 4, 4]
                 expanded_starts = torch.repeat_interleave(
-                    common_attn_metadata.query_start_loc[:num_decodes], decode_lens
+                    common_attn_metadata.query_start_loc[:num_decodes],
+                    decode_lens,
+                    output_size=actual_expanded,
                 )
 
                 # [0, 1, 2, 0, 0, 1, 2, 3]
@@ -407,7 +409,9 @@ class DeepseekV32IndexerMetadataBuilder(AttentionMetadataBuilder):
                 # Give each of the flattened entries the same block table row as the
                 # original request.
                 self.expanded_block_table_buffer[:actual_expanded] = (
-                    torch.repeat_interleave(block_table, decode_lens, dim=0)
+                    torch.repeat_interleave(
+                        block_table, decode_lens, dim=0, output_size=actual_expanded
+                    )
                 )
                 if actual_expanded < num_decode_tokens:
                     self.expanded_block_table_buffer[