Add shifted sparse attention (#973) [skip-ci]

* Add s2_attn to hijack flash code

* Refactor code to account for s2_attn

* Add test for models utils

* Add ``s2_attention`` option to llama configs

* Add ``s2_attention`` option to README config

* Format code to appease linter

* chore: lint

* Remove xpos and llama-landmark [bad merge]

* add e2e smoke tests for shifted sparse attention

* remove stray patch from merge

* update yml with link to paper for s2_attention/longlora

* fix assertion check for full fine tune

* increase sequence len for tests and PR feedback updates

* reduce context len to 16k for tests

* reduce context len to 16k for tests

* reduce batch size for larger context len and udpate test to check message

* fix test for message

---------

Co-authored-by: joecummings <jrcummings@devvm050.nha0.facebook.com>
Co-authored-by: Wing Lian <wing.lian@gmail.com>

examples/code-llama/13b/lora.yml

@@ -52,6 +52,7 @@ local_rank:
 logging_steps: 1
 xformers_attention:
 flash_attention: true
+s2_attention:
 
 warmup_steps: 10
 evals_per_epoch: 4

examples/code-llama/34b/lora.yml

@@ -52,6 +52,7 @@ local_rank:
 logging_steps: 1
 xformers_attention:
 flash_attention: true
+s2_attention:
 
 warmup_steps: 10
 evals_per_epoch: 4

examples/code-llama/7b/lora.yml

@@ -52,6 +52,7 @@ local_rank:
 logging_steps: 1
 xformers_attention:
 flash_attention: true
+s2_attention:
 
 warmup_steps: 10
 evals_per_epoch: 4