Fix unsigned integer underflow

Passing --max-batch-size to the launcher actually had no effect
because after a few requests the max_size passed to State::next_batch
would underflow becoming a largo positive number.

In the scheduler, as soon as the cached batch size reached the
max_batch_size the max_size passed to next_batch becomes 0.
Since the only check in that funcion is
```
if Some(batch_requests.len()) == max_size {
    break;
}
```
and it's called after the `batch_requests.len()` has
become 1, it doesn't do anything to prevent more than 0
requests from being batched.

Now we have cached batch in the server that is large than
max_batch_size and `max_size - batch_size as usize`
underflows.

Signed-off-by: Max de Bayser <mbayser@br.ibm.com>

router/src/infer/v2/queue.rs

@@ -205,6 +205,13 @@ impl State {
             }
         }
 
+        if let Some(max_size) = max_size {
+            if max_size == 0 {
+                tracing::debug!("No capacity");
+                return None;
+            }
+        }
+
         // Pad prefill_token_budget to be a multiple of block size
         let prefill_token_budget =
             ((prefill_token_budget + self.block_size - 1) / self.block_size) * self.block_size;
@@ -297,7 +304,7 @@ impl State {
             batch_entries.insert(id, entry);
 
             // Check if max_size
-            if Some(batch_requests.len()) == max_size {
+            if Some(batch_requests.len()) >= max_size {
                 break;
             }
         }

router/src/infer/v2/scheduler.rs

@@ -161,7 +161,10 @@ pub(crate) async fn batching_task(
                 };
 
                 let token_budget = max_batch_total_tokens.saturating_sub(batch_max_tokens);
-                let max_size = max_batch_size.map(|max_size| max_size - batch_size as usize);
+                let max_size = max_batch_size.map(|max_size| {
+                    if batch_size as usize > max_size { 0 } else { max_size - batch_size as usize }
+                });
+
 
                 // Try to get a new batch
                 if let Some((mut new_entries, new_batch, span)) = queue