feat: added README

README.md

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+# BERT with Flash-Attention
+### Installing dependencies
+To run the model on GPU, you need to install Flash Attention. 
+You may either install from pypi (which may not work with fused-dense), or from source.
+To install from source, clone the GitHub repository:
+```console
+git clone git@github.com:Dao-AILab/flash-attention.git
+```
+The code provided here should work with commit `43950dd`.
+Change to the cloned repo and install:
+```console
+cd flash-attention && python setup.py install
+```
+This will compile the flash-attention kernel, which will take some time.
+
+If you would like to use fused MLPs (e.g. to use activation checkpointing),
+you may install fused-dense also from source:
+```console
+cd csrc/fused_dense_lib && python setup.py install
+```
+
+
+### Configuration
+The config adds some new parameters:
+- `use_flash_attn`: If `True`, always use flash attention. If `None`, use flash attention when GPU is available. If `False`, never use flash attention (works on CPU).
+- `window_size`: Size (left and right) of the local attention window. If `(-1, -1)`, use global attention
+- `dense_seq_output`: If true, we only need to pass the hidden states for the masked out token (around 15%) to the classifier heads. I set this to true for pretraining.
+- `fused_mlp`: Whether to use fused-dense. Useful to reduce VRAM in combination with activation checkpointing
+- `mlp_checkpoint_lvl`: One of `{0, 1, 2}`. Increasing this increases the amount of activation checkpointing within the MLP. Keep this at 0 for pretraining and use gradient accumulation instead. For embedding training, increase this as much as needed.
+- `last_layer_subset`: If true, we only need the compute the last layer for a subset of tokens. I left this to false.
+- `use_qk_norm`: Whether or not to use QK-normalization
+- `num_loras`: Number of LoRAs to use when initializing a `BertLoRA` model. Has no effect on other models.
+