Add --prompt_file flag to the eagle3 runner

Reads the prompt text from a file instead of --prompt, mirroring the
gemma4-31B runner. The prompt still goes through the normal encode +
chat-template + BOS path, so this is independent of tokenization.

Authored with assistance from Claude Code.


ghstack-source-id: 4c633c4
ghstack-comment-id: 4734207725
Pull-Request: #20348

Author: digantdesai

examples/models/eagle3/main.cpp

@@ -75,6 +75,7 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstring>
+#include <fstream>
 #include <string>
 #include <vector>
 
@@ -102,6 +103,10 @@ DEFINE_string(model_path, "", "Speculator model.pte path.");
 DEFINE_string(data_path, "", "Tensor data (.ptd) path for the CUDA backend.");
 DEFINE_string(tokenizer_path, "", "HuggingFace tokenizer.json path.");
 DEFINE_string(prompt, "Explain why the sky is blue.", "Prompt text.");
+DEFINE_string(
+    prompt_file,
+    "",
+    "Read the prompt text from this file instead of --prompt.");
 DEFINE_bool(raw_prompt, false, "Skip the Gemma 4 IT chat template.");
 DEFINE_int32(max_new_tokens, 128, "Maximum tokens to generate.");
 DEFINE_int32(bos_id, 2, "BOS token id (-1 to skip; Gemma convention: 2).");
@@ -314,6 +319,16 @@ int main(int argc, char** argv) {
   }
 
   std::string prompt_text = FLAGS_prompt;
+  if (!FLAGS_prompt_file.empty()) {
+    std::ifstream f(FLAGS_prompt_file);
+    if (!f) {
+      ET_LOG(
+          Error, "Failed to open prompt file: %s", FLAGS_prompt_file.c_str());
+      return 1;
+    }
+    prompt_text = std::string(
+        std::istreambuf_iterator<char>(f), std::istreambuf_iterator<char>());
+  }
   if (!FLAGS_raw_prompt) {
     prompt_text = FLAGS_chat_prefix + prompt_text + FLAGS_chat_suffix;
   }