Add simple inference FLOP counter to calc_transformer_flops.py (#31)

* add --infer arg to flops calculator

* add comment

* fix comment

* Update calc_transformer_flops.py

calc/calc_transformer_flops.py

@@ -66,11 +66,13 @@ def config_parser():
                         action='store_false',
                         help='Whether Megatron-style activation checkpointing is being used',
                         dest='checkpoint_activations')
+    parser.add_argument("--infer", "-i", 
+                        action='store_true',
+                        help='Pass to calculate FLOPs for inference-only workload (no backward pass)')
     return parser
 
 # calculates the flops of a model given its hparams
 def calc_params(args):
-
     assert args.topk <= args.num_experts, "You cannot route to more experts than you have!"
     assert args.num_layers % args.expert_interval == 0, "Require for simplicity that we don't have hanging dense layers"
 
@@ -82,6 +84,10 @@ def calc_params(args):
     iter_factor = 3
     if args.checkpoint_activations:
         iter_factor += 1
+    # If inference-only, no bwd pass or activation ckpting necessary
+    # This assumes simply running a single forward pass ('prefill' stage of decoding) and no subsequent autoregressively generated tokens.
+    if args.infer:
+        iter_factor = 1
 
     qkv_flops = int(iter_factor * 2 * (1 + 2 * args.kv_size_ratio) * args.num_layers * args.tokens * args.hidden_size * args.hidden_size)
     attention_matrix_flops = iter_factor * 2 * args.num_layers * args.tokens * args.sequence_length * args.hidden_size