feat: cheap ppl metric (#3317)

* Import math and compute perplexity from loss values

* lint

* coderabbit changes

* lint

* fix: add rounding to ppl

---------

Co-authored-by: NanoCode012 <nano@axolotl.ai>

src/axolotl/core/trainers/base.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import math
 import os
 from collections import defaultdict
 from functools import partial, wraps
@@ -615,6 +616,17 @@ class AxolotlTrainer(
                 )
             logs[key] = round(fn(values).item(), 4)
 
+        if "loss" in logs:
+            try:
+                logs["ppl"] = round(math.exp(logs["loss"]), 4)
+            except OverflowError:
+                logs["ppl"] = float("inf")
+        if "eval_loss" in logs:
+            try:
+                logs["eval_ppl"] = round(math.exp(logs["eval_loss"]), 4)
+            except OverflowError:
+                logs["eval_ppl"] = float("inf")
+
         if is_main_process():
             # Add memory usage
             try: