[Docs] Add BERT pre-training experiment documentation to train.md

https://github.com/tensorflow/models/issues/10074

PiperOrigin-RevId: 463195812

official/nlp/configs/experiments/wiki_books_pretrain.yaml

+task:
+  init_checkpoint: ''
+  model:
+    cls_heads: [{activation: tanh, cls_token_idx: 0, dropout_rate: 0.1, inner_dim: 768, name: next_sentence, num_classes: 2}]
+  train_data:
+    drop_remainder: true
+    global_batch_size: 512
+    input_path: '[Your proceed wiki data path]*,[Your proceed books data path]*'
+    is_training: true
+    max_predictions_per_seq: 76
+    seq_length: 512
+    use_next_sentence_label: true
+    use_position_id: false
+    use_v2_feature_names: true
+  validation_data:
+    drop_remainder: false
+    global_batch_size: 512
+    input_path: '[Your proceed wiki data path]-00000-of-00500,[Your proceed books data path]-00000-of-00500'
+    is_training: false
+    max_predictions_per_seq: 76
+    seq_length: 512
+    use_next_sentence_label: true
+    use_position_id: false
+    use_v2_feature_names: true
+trainer:
+  checkpoint_interval: 20000
+  max_to_keep: 5
+  optimizer_config:
+    learning_rate:
+      polynomial:
+        cycle: false
+        decay_steps: 1000000
+        end_learning_rate: 0.0
+        initial_learning_rate: 0.0001
+        power: 1.0
+      type: polynomial
+    optimizer:
+      type: adamw
+    warmup:
+      polynomial:
+        power: 1
+        warmup_steps: 10000
+      type: polynomial
+  steps_per_loop: 1000
+  summary_interval: 1000
+  train_steps: 1000000
+  validation_interval: 1000
+  validation_steps: 64

official/nlp/docs/train.md

@@ -229,6 +229,73 @@ python3 train.py \
 
 ```
 
+### Pre-train a BERT from scratch
+
 </details>
 
-Note: More examples about pre-training will come soon.
+This example pre-trains a BERT model with Wikipedia and Books datasets used by
+the original BERT paper.
+The [BERT repo](https://github.com/tensorflow/models/blob/master/official/nlp/data/create_pretraining_data.py)
+contains detailed information about the Wikipedia dump and
+[BookCorpus](https://yknzhu.wixsite.com/mbweb). Of course, the pre-training
+recipe is generic and you can apply the same recipe to your own corpus.
+
+Please use the script
+[`create_pretraining_data.py`](https://github.com/tensorflow/models/blob/master/official/nlp/data/create_pretraining_data.py)
+which is essentially branched from [BERT research repo](https://github.com/google-research/bert)
+to get processed pre-training data and it adapts to TF2 symbols and python3
+compatibility.
+
+Running the pre-training script requires an input and output directory, as well
+as a vocab file. Note that `max_seq_length` will need to match the sequence
+length parameter you specify when you run pre-training.
+
+```shell
+export WORKING_DIR='local disk or cloud location'
+export BERT_DIR='local disk or cloud location'
+python models/official/nlp/data/create_pretraining_data.py \
+  --input_file=$WORKING_DIR/input/input.txt \
+  --output_file=$WORKING_DIR/output/tf_examples.tfrecord \
+  --vocab_file=$BERT_DIR/wwm_uncased_L-24_H-1024_A-16/vocab.txt \
+  --do_lower_case=True \
+  --max_seq_length=512 \
+  --max_predictions_per_seq=76 \
+  --masked_lm_prob=0.15 \
+  --random_seed=12345 \
+  --dupe_factor=5
+```
+
+Then, you can update the yaml configuration file, e.g.
+`configs/experiments/wiki_books_pretrain.yaml` to specify your data paths and
+update masking-related hyper parameters to match with your specification for 
+the pretraining data. When your data have multiple shards, you can
+use `*` to include multiple files.
+
+To train different BERT sizes, you need to adjust:
+
+```
+model:
+  cls_heads: [{activation: tanh, cls_token_idx: 0, dropout_rate: 0.1, inner_dim: 768, name: next_sentence, num_classes: 2}]
+```
+
+to match the hidden dimensions.
+
+Then, you can start the training and evaluation jobs, which runs the
+[`bert/pretraining`](https://github.com/tensorflow/models/blob/master/official/nlp/configs/pretraining_experiments.py#L51)
+experiment:
+
+```shell
+export OUTPUT_DIR=gs://some_bucket/my_output_dir
+export PARAMS=$PARAMS,runtime.distribution_strategy=tpu
+
+python3 train.py \
+ --experiment=bert/pretraining \
+ --mode=train_and_eval \
+ --model_dir=$OUTPUT_DIR \
+ --config_file=configs/models/bert_en_uncased_base.yaml \
+ --config_file=configs/experiments/wiki_books_pretrain.yaml \
+ --tpu=${TPU_NAME} \
+ --params_override=$PARAMS
+```
+
+Note: More examples about pre-training with TFDS datesets will come soon.