add comment documenting how to reproduce python reference exactly

train_gpt2.cu

@@ -24,6 +24,13 @@ Also we're using TinyStories here for example as it is a bigger dataset
 
 Example launch using bfloat16 on 4 GPUs, same as above:
 mpirun -np 4 ./train_gpt2cu -b 8 -v 200 -s 200 -i data/TinyStories
+
+If you'd like to see train_gpt2.cu produce identical results to
+`python train_gpt2.py`, you can run it like this:
+make train_gpt2cu PRECISION=FP32
+./train_gpt2cu -b 4 -t 64 -l 1e-4 -v 200 -s 200 -a 1 -x 10 -f 0
+This reads & runs in fp32, B=4, T=64, LR=1e-4, val/sample never (200),
+-a 1 is "overfit single batch", -x 10 is 10 iterations, and -f 0 disables tf32
 */
 
 #include <stdio.h>