Fix LLVM-12 UB in generate_proposals_op.cc (pytorch#66009)

Summary:
Pull Request resolved: pytorch#66009

Fixes
```
test_trace_c10_ops (jit.test_tracer.TestTracer) ... third-party-buck/platform009/build/eigen/include/Eigen/src/Core/Block.h:374:24: runtime error: applying non-zero offset 4 to null pointer
    #0 0x7f5228f72227 in Eigen::internal::BlockImpl_dense<Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> >, -1, -1, false, true>::BlockImpl_dense(Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> >&, long, long, long, long) third-party-buck/platform009/build/eigen/include/Eigen/src/Core/Block.h:374
    #1 0x7f5228f7212c in Eigen::BlockImpl<Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> >, -1, -1, false, Eigen::Dense>::BlockImpl(Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> >&, long, long, long, long) third-party-buck/platform009/build/eigen/include/Eigen/src/Core/Block.h:166
    #2 0x7f5228f720dc in Eigen::Block<Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> >, -1, -1, false>::Block(Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> >&, long, long, long, long) third-party-buck/platform009/build/eigen/include/Eigen/src/Core/Block.h:142
    #3 0x7f5229b0e059 in Eigen::DenseBase<Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> > >::FixedBlockXpr<internal::get_fixed_value<int>::value, internal::get_fixed_value<long>::value>::Type Eigen::DenseBase<Eigen::Map<Eigen::Array<float, -1, -1, 1, -1, -1>, 0, Eigen::Stride<0, 0> > >::block<int, long>(long, long, int, long) third-party-buck/platform009/build/eigen/include/Eigen/src/Core/../plugins/BlockMethods.h:98
    #4 0x7f5229b0c5ca in caffe2::GenerateProposalsOp<caffe2::CPUContext>::RunOnDevice() caffe2/caffe2/operators/generate_proposals_op.cc:348
```
Also cleans up some data type and const issues around the area.

Test Plan: Sandcastle

Reviewed By: xush6528

Differential Revision: D31343046

fbshipit-source-id: fd9096c8e47a0aad529c72fd313f64ca98dcb80b

Author: r-barnes

caffe2/operators/generate_proposals_op.cc

@@ -328,16 +328,21 @@ bool GenerateProposalsOp<CPUContext>::RunOnDevice() {
   }
 
   int roi_counts = 0;
-  for (int i = 0; i < num_images; i++) {
+  for (int64_t i = 0; i < num_images; i++) {
     roi_counts += im_boxes[i].rows();
   }
-  // NOLINTNEXTLINE(cppcoreguidelines-narrowing-conversions,bugprone-narrowing-conversions)
-  const int roi_col_count = box_dim + 1;
-  auto* out_rois = Output(0, {roi_counts, roi_col_count}, at::dtype<float>());
-  auto* out_rois_probs = Output(1, {roi_counts}, at::dtype<float>());
+
+  const int64_t roi_col_count = box_dim + 1;
+  auto *const out_rois = Output(0, {roi_counts, roi_col_count}, at::dtype<float>());
+  auto *const out_rois_probs = Output(1, {roi_counts}, at::dtype<float>());
+
+  if(roi_counts == 0){
+    return true;
+  }
+
   float* out_rois_ptr = out_rois->template mutable_data<float>();
   float* out_rois_probs_ptr = out_rois_probs->template mutable_data<float>();
-  for (int i = 0; i < num_images; i++) {
+  for (int64_t i = 0; i < num_images; i++) {
     const ERArrXXf& im_i_boxes = im_boxes[i];
     const EArrXf& im_i_probs = im_probs[i];
     int csz = im_i_boxes.rows();