fix runner max seq len (#2688)

Summary:
Pull Request resolved: #2688

Max seq len arg when not passed uses the max seq len from the model.
THis means, num tokens generated should be equal to kv cache size.
However, generate loop tries to generate one more token because pos, 0 based
index, is taken for num tokens

Reviewed By: mergennachin, digantdesai

Differential Revision: D55369776

fbshipit-source-id: 7beb38177a23449649e96184b0b0a0bb507c199f

Author: kimishpatel

examples/models/llama2/runner/runner.cpp

@@ -238,7 +238,7 @@ Error Runner::generate(
   }
 
   // create a 1xN int tensor with next as value
-  while (pos < seq_len) {
+  while (pos + 1 < seq_len) {
     // ET_LOG(Info, "Generating step %d...", pos);
     // set the current token in the tensor
     std::vector<EValue> inputs;
@@ -339,11 +339,11 @@ Error Runner::generate(
   timers_.inference_end_ms = util::time_in_ms();
   printf("\n");
 
-  if (pos == seq_len) {
+  if (pos + 1 == seq_len) {
     ET_LOG(Info, "Sequence length (%i tokens) reached!", seq_len);
   }
 
-  timers_.printReport(num_prompt_tokens, pos - num_prompt_tokens);
+  timers_.printReport(num_prompt_tokens, (pos + 1) - num_prompt_tokens);
 
   delete[] prompt_tokens;
   return Error::Ok;