Fix the integer overflow problem of sequence2batch (#22479)

Fix the  integer overflow problem in the op of sequence2batch, change the int32_t to size_t，
In the /paddle/fluid/operators/math/sequence2batch.h#L122.

paddle/fluid/operators/math/sequence2batch.h

@@ -50,11 +50,11 @@ class LoDTensor2BatchFunctor {
   //           seq_info[3] = {(4, 5, 1), (0, 4, 0), (9, 3, 2)}
   //
   struct SeqInfo {
-    SeqInfo(int start, int length, int seq_idx)
+    SeqInfo(size_t start, size_t length, size_t seq_idx)
         : start(start), length(length), seq_idx(seq_idx) {}
-    int start;
-    int length;
-    int seq_idx;
+    size_t start;
+    size_t length;
+    size_t seq_idx;
   };
 
  public:
@@ -82,7 +82,7 @@ class LoDTensor2BatchFunctor {
 
     std::vector<SeqInfo> seq_info;
     for (size_t seq_id = 0; seq_id < lod.size() - 1; ++seq_id) {
-      int length = lod[seq_id + 1] - lod[seq_id];
+      size_t length = lod[seq_id + 1] - lod[seq_id];
       seq_info.emplace_back(lod[seq_id], length, seq_id);
     }
 
@@ -118,8 +118,8 @@ class LoDTensor2BatchFunctor {
     batch_lods.emplace_back(std::vector<size_t>{0});
 
     // batch_lods[0] is the start positions for batch LoDTensor
-    int max_seqlen = seq_info[0].length;
-    batch_lods[0].resize(static_cast<size_t>(max_seqlen + 1));
+    size_t max_seqlen = seq_info[0].length;
+    batch_lods[0].resize(max_seqlen + 1);
     // batch_lods[1] is the raw index in the input LoDTensor
     batch_lods[1].resize(static_cast<size_t>(lod_tensor.dims()[0]));
     // batch_lods[2] is the sort order for the input LoDTensor.
@@ -128,11 +128,11 @@ class LoDTensor2BatchFunctor {
     size_t* batch_starts = batch_lods[0].data();
     size_t* seq2batch_idx = batch_lods[1].data();
     batch_starts[0] = 0;
-    for (int n = 0; n < max_seqlen; n++) {
-      auto batch_id = static_cast<int>(batch_starts[n]);
+    for (size_t n = 0; n < max_seqlen; n++) {
+      size_t batch_id = batch_starts[n];
       for (size_t i = 0; i < seq_info.size(); ++i) {
-        int seq_len = seq_info[i].length;
-        int start = seq_info[i].start;
+        size_t seq_len = seq_info[i].length;
+        size_t start = seq_info[i].start;
         if (n < seq_len) {
           seq2batch_idx[batch_id] =
               is_reverse ? start + seq_len - 1 - n : start + n;
@@ -141,7 +141,7 @@ class LoDTensor2BatchFunctor {
           break;
         }
       }
-      batch_starts[n + 1] = static_cast<size_t>(batch_id);
+      batch_starts[n + 1] = batch_id;
     }
     size_t* seq_order = batch_lods[2].data();
     for (size_t i = 0; i < seq_info.size(); ++i) {