[Frontend] Add TRITON_PRINT_AUTOTUNING flag (#3411)

When the `TRITON_PRINT_AUTOTUNING` envvar is set to 1, we print a message after autotuning a kernel.

For example:

```
Triton autotuning for function JITFunction(__main__:fused_add_rmsnorm_triton) finished after 4.15s; best config selected: BLOCK_N_SIZE: 512, num_warps: 8, num_ctas: 1, num_stages: 3;
```

Co-authored-by: Justin Lebar <justin.lebar@gmail.com>

Author: bringlein

python/triton/runtime/autotuner.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import builtins
+import os
 import time
 from typing import Dict
 
@@ -109,6 +110,7 @@ def kernel_call():
 
     def run(self, *args, **kwargs):
         self.nargs = dict(zip(self.arg_names, args))
+        used_cached_result = True
         if len(self.configs) > 1:
             all_args = {**self.nargs, **kwargs}
             _args = []
@@ -122,6 +124,7 @@ def run(self, *args, **kwargs):
             key = tuple(key)
             if key not in self.cache:
                 # prune configs
+                used_cached_result = False
                 pruned_configs = self.prune_configs(kwargs)
                 bench_start = time.time()
                 timings = {config: self._bench(*args, config=config, **kwargs) for config in pruned_configs}
@@ -134,6 +137,9 @@ def run(self, *args, **kwargs):
         else:
             config = self.configs[0]
         self.best_config = config
+        if os.getenv("TRITON_PRINT_AUTOTUNING", None) == "1" and not used_cached_result:
+            print(f"Triton autotuning for function {self.fn} finished after "
+                  f"{self.bench_time:.2f}s; best config selected: {self.best_config};")
         full_nargs = {**self.nargs, **kwargs, **self.best_config.kwargs}
         if config.pre_hook is not None:
             config.pre_hook(full_nargs)