Use eos token for padding lazily (#369)

lib/bumblebee/text/pre_trained_tokenizer.ex

@@ -131,10 +131,7 @@ defmodule Bumblebee.Text.PreTrainedTokenizer do
       special_tokens: %{
         unk: "<|endoftext|>",
         bos: "<|endoftext|>",
-        eos: "<|endoftext|>",
-        # CodeGen doesn't originally have a pad token, however when necessary
-        # we pad with the EOS token
-        pad: "<|endoftext|>"
+        eos: "<|endoftext|>"
       }
     },
     distilbert: %{
@@ -152,20 +149,14 @@ defmodule Bumblebee.Text.PreTrainedTokenizer do
       special_tokens: %{
         unk: "<|endoftext|>",
         bos: "<|endoftext|>",
-        eos: "<|endoftext|>",
-        # GPT-NeoX doesn't originally have a pad token, however when necessary
-        # we pad with the EOS token
-        pad: "<|endoftext|>"
+        eos: "<|endoftext|>"
       }
     },
     gpt2: %{
       special_tokens: %{
         unk: "<|endoftext|>",
         bos: "<|endoftext|>",
-        eos: "<|endoftext|>",
-        # GPT-2 doesn't originally have a pad token, however when necessary
-        # we pad with the EOS token
-        pad: "<|endoftext|>"
+        eos: "<|endoftext|>"
       }
     },
     layout_lm: %{
@@ -175,10 +166,7 @@ defmodule Bumblebee.Text.PreTrainedTokenizer do
       special_tokens: %{
         eos: "</s>",
         unk: "<unk>",
-        sep: "</s>",
-        # Llama doesn't originally have a pad token, however when necessary
-        # we pad with the EOS token
-        pad: "</s>"
+        sep: "</s>"
       }
     },
     mbart: %{
@@ -275,8 +263,11 @@ defmodule Bumblebee.Text.PreTrainedTokenizer do
   def apply(tokenizer, input) do
     input = List.wrap(input)
 
+    # Some tokenizers don't specify a PAD token, in which case we use
+    # the EOS token for padding by default
     pad_token =
       tokenizer.special_tokens[:pad] ||
+        tokenizer.special_tokens[:eos] ||
         raise ArgumentError,
               "expected the tokenizer to defined a padding token, but none was found"