Script to export 🤗 models (#4723)

Summary:
bypass-github-export-checks

[Done] ~~Require PR [Make StaticCache configurable at model construct time](https://github.com/huggingface/transformers/pull/32830) in order to export, lower and run the 🤗 model OOTB.~~
[Done] ~~Require https://github.com/huggingface/transformers/pull/33303 or https://github.com/huggingface/transformers/pull/33287 to be merged to 🤗 `transformers` to resolve the export issue introduced by https://github.com/huggingface/transformers/pull/32543~~

-----------

Now we can take the integration point from 🤗 `transformers` to lower compatible models to ExecuTorch OOTB.
  - This PR creates a simple script with recipe of XNNPACK.
  - This PR also created a secret `EXECUTORCH_HT_TOKEN` to allow download checkpoints in the CI
  - This PR connects the 🤗 "Export to ExecuTorch" e2e workflow to ExecuTorch CI

### Instructions to run the demo:

1. Run the export_hf_model.py to lower gemma-2b to ExecuTorch:
```
python -m extension.export_util.export_hf_model -hfm "google/gemma-2b" # The model is exported statical dims with static KV cache
```
2. Run the tokenizer.py to generate the binary format for ExecuTorch runtime:
```
python -m extension.llm.tokenizer.tokenizer -t <path_to_downloaded_gemma_checkpoint_dir>/tokenizer.model -o tokenizer.bin
```
3. Build llm runner by following this guide [step 4](https://github.com/pytorch/executorch/tree/main/examples/models/llama2#step-4-run-on-your-computer-to-validate)

4. Run the lowered model
```
cmake-out/examples/models/llama2/llama_main --model_path=gemma.pte --tokenizer_path=tokenizer.bin --prompt="My name is"
```
OOTB output and perf
```
I 00:00:00.003110 executorch:cpuinfo_utils.cpp:62] Reading file /sys/devices/soc0/image_version
I 00:00:00.003360 executorch:cpuinfo_utils.cpp:78] Failed to open midr file /sys/devices/soc0/image_version
I 00:00:00.003380 executorch:cpuinfo_utils.cpp:158] Number of efficient cores 4
I 00:00:00.003384 executorch:main.cpp:65] Resetting threadpool with num threads = 6
I 00:00:00.014716 executorch:runner.cpp:51] Creating LLaMa runner: model_path=gemma.pte, tokenizer_path=tokenizer_gemma.bin
I 00:00:03.065359 executorch:runner.cpp:66] Reading metadata from model
I 00:00:03.065391 executorch:metadata_util.h:43] get_n_bos: 1
I 00:00:03.065396 executorch:metadata_util.h:43] get_n_eos: 1
I 00:00:03.065399 executorch:metadata_util.h:43] get_max_seq_len: 123
I 00:00:03.065402 executorch:metadata_util.h:43] use_kv_cache: 1
I 00:00:03.065404 executorch:metadata_util.h:41] The model does not contain use_sdpa_with_kv_cache method, using default value 0
I 00:00:03.065405 executorch:metadata_util.h:43] use_sdpa_with_kv_cache: 0
I 00:00:03.065407 executorch:metadata_util.h:41] The model does not contain append_eos_to_prompt method, using default value 0
I 00:00:03.065409 executorch:metadata_util.h:43] append_eos_to_prompt: 0
I 00:00:03.065411 executorch:metadata_util.h:41] The model does not contain enable_dynamic_shape method, using default value 0
I 00:00:03.065412 executorch:metadata_util.h:43] enable_dynamic_shape: 0
I 00:00:03.130388 executorch:metadata_util.h:43] get_vocab_size: 256000
I 00:00:03.130405 executorch:metadata_util.h:43] get_bos_id: 2
I 00:00:03.130408 executorch:metadata_util.h:43] get_eos_id: 1
My name is Melle. I am a 20 year old girl from Belgium. I am living in the southern part of Belgium. I am 165 cm tall and I weigh 45kg. I like to play sports like swimming, running and playing tennis. I am very interested in music and I like to listen to classical music. I like to sing and I can play the piano. I would like to go to the USA because I like to travel a lot. I am looking for a boy from the USA who is between 18 and 25 years old. I
PyTorchObserver {"prompt_tokens":4,"generated_tokens":118,"model_load_start_ms":1723685715497,"model_load_end_ms":1723685718612,"inference_start_ms":1723685718612,"inference_end_ms":1723685732965,"prompt_eval_end_ms":1723685719087,"first_token_ms":1723685719087,"aggregate_sampling_time_ms":182,"SCALING_FACTOR_UNITS_PER_SECOND":1000}
I 00:00:17.482472 executorch:stats.h:70] 	Prompt Tokens: 4    Generated Tokens: 118
I 00:00:17.482475 executorch:stats.h:76] 	Model Load Time:		3.115000 (seconds)
I 00:00:17.482481 executorch:stats.h:86] 	Total inference time:		14.353000 (seconds)		 Rate: 	8.221278 (tokens/second)
I 00:00:17.482483 executorch:stats.h:94] 		Prompt evaluation:	0.475000 (seconds)		 Rate: 	8.421053 (tokens/second)
I 00:00:17.482485 executorch:stats.h:105] 		Generated 118 tokens:	13.878000 (seconds)		 Rate: 	8.502666 (tokens/second)
I 00:00:17.482486 executorch:stats.h:113] 	Time to first generated token:	0.475000 (seconds)
I 00:00:17.482488 executorch:stats.h:120] 	Sampling time over 122 tokens:	0.182000 (seconds)
```

Pull Request resolved: https://github.com/pytorch/executorch/pull/4723

Reviewed By: huydhn, kirklandsign

Differential Revision: D62543933

Pulled By: guangy10

fbshipit-source-id: 00401a39ba03d7383e4b284d25c8fc62a6695b34