Merge branch 'master_params_sharing' into 'master'

Parameters sharing

See merge request ADLR/megatron-lm!82

megatron/arguments.py

@@ -89,6 +89,14 @@ def parse_args(extra_args_provider=None, defaults={},
         assert args.min_lr <= args.lr
     if args.save is not None:
         assert args.save_interval is not None
+    # Parameters sharing does not work with torch DDP.
+    if (args.num_unique_layers is not None) and (args.num_layers is not None):
+        assert args.num_unique_layers <= args.num_layers
+        assert args.num_layers % args.num_unique_layers == 0, \
+            'num-layers should be divisible by num-unique-layers.'
+        if args.num_unique_layers < args.num_layers:
+            assert args.DDP_impl == 'local', \
+                'torch-DDP does not work with parameters sharing.'
 
     _print_args(args)
     return args
@@ -116,6 +124,16 @@ def _add_network_size_args(parser):
 
     group.add_argument('--num-layers', type=int, default=None,
                        help='Number of transformer layers.')
+    group.add_argument('--num-unique-layers', type=int, default=None,
+                       help='Number of unique transformer layers. '
+                       '`num-layers` should be divisible by this value.')
+    group.add_argument('--param-sharing-style', default='grouped',
+                       choices=['grouped', 'spaced'],
+                       help='Ordering of the shared parameters. For example, '
+                       'for a `num-layers`=4 and `--num-unique-layers`=2, '
+                       'we will have the following ordering for two unique '
+                       'layers 1 and 2: '
+                       '    grouped: [1, 2, 1, 2] and spaced: [1, 1, 2, 2].')
     group.add_argument('--hidden-size', type=int, default=None,
                        help='Tansformer hidden size.')
     group.add_argument('--num-attention-heads', type=int, default=None,

megatron/model/transformer.py

@@ -360,34 +360,60 @@ class ParallelTransformer(MegatronModule):
         self.checkpoint_activations = args.checkpoint_activations
         self.checkpoint_num_layers = args.checkpoint_num_layers
 
-        def get_layer(layer_number):
+        # Number of layers:
+        self.num_layers = args.num_layers
+        self.num_unique_layers = args.num_unique_layers
+        if self.num_unique_layers is None:
+            self.num_unique_layers = self.num_layers
+        assert self.num_layers % self.num_unique_layers == 0, \
+            'number of layers should be divisible by number of unique layers'
+        self.param_sharing_style = args.param_sharing_style
+
+        # Transformer layers.
+        def build_layer(layer_number):
             return ParallelTransformerLayer(
                 attention_mask_func, mlp_activation_func,
                 init_method, output_layer_init_method, layer_number)
-
-        # Transformer layers.
         self.layers = torch.nn.ModuleList(
-            [get_layer(i + 1) for i in range(args.num_layers)])
+            [build_layer(i + 1) for i in range(self.num_unique_layers)])
+
+        # Print layer ordering.
+        if self.num_layers != self.num_unique_layers:
+            if torch.distributed.get_rank() == 0:
+                print('> will be using the following layer ordering:')
+                for i in range(self.num_layers):
+                    print('   layer id: {:3d} --> unique layer id: '
+                          '{:3d}'.format(i, self._get_layer_index(i)),
+                          flush=True)
 
         # Final layer norm before output.
         self.final_layernorm = LayerNorm(
             args.hidden_size,
             eps=args.layernorm_epsilon)
 
+    def _get_layer_index(self, layer_number):
+        if self.param_sharing_style == 'grouped':
+            return layer_number % self.num_unique_layers
+        if self.param_sharing_style == 'spaced':
+            return layer_number // (self.num_layers // self.num_unique_layers) 
+        assert False, 'should not be here'
+
+    def _get_layer(self, layer_number):
+        return self.layers[self._get_layer_index(layer_number)]
+
     def _checkpointed_forward(self, hidden_states, attention_mask):
         """Forward method with activation checkpointing."""
         def custom(start, end):
             def custom_forward(*inputs):
-                layers_ = self.layers[start:end]
                 x_ = inputs[0]
-                for layer in layers_:
+                for index in range(start, end):
+                    layer = self._get_layer(index)
                     x_ = layer(x_, inputs[1])
                 return x_
             return custom_forward
 
         l = 0
-        num_layers = len(self.layers)
-        while l < num_layers:
+        while l < self.num_layers:
             hidden_states = mpu.checkpoint(
                 custom(l, l + self.checkpoint_num_layers),
                 hidden_states, attention_mask)
@@ -414,10 +440,11 @@ class ParallelTransformer(MegatronModule):
         else:
             if get_key_value:
                 presents = []
-            for i, layer in enumerate(self.layers):
+            for index in range(self.num_layers):
+                layer = self._get_layer(index)
                 past = None
                 if layer_past is not None:
-                    past = layer_past[i]
+                    past = layer_past[index]
                 hidden_states = layer(hidden_states,
                                       attention_mask,
                                       layer_past=past,