Tranformers documentation translation to Italian

Hi!

Let's bring the documentation to all the Italian-speaking community :)

Who would want to translate? Please follow the 🤗 [TRANSLATING guide](https://github.com/huggingface/transformers/blob/main/docs/TRANSLATING.md). Here is a list of the files ready for translation. Let us know here if you'd like to translate any and we'll add your name to the list.

Some notes:

- Please translate using an informal tone (imagine you are talking with a friend about transformers 🤗). For example, use Tu instead of Lei.
- Please translate in a gender-neutral way.
- Add your translations to the folder called `it` inside the [source folder](https://github.com/huggingface/transformers/tree/main/docs/source).
- Register your translation in [it/_toctree.yml](https://github.com/huggingface/transformers/blob/main/docs/source/it/_toctree.yml); please follow the order of the [English version](https://github.com/huggingface/transformers/blob/main/docs/source/en/_toctree.yml).
- Once you're finished, open a pull request and tag this issue by including #issue-number in the description, where issue-number is the number of this issue.
- 🙋 If you'd like others to help you with the translation, you can also post in the 🤗 [forums](https://discuss.huggingface.co/).

## Get Started section
- [x] [index.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/index.mdx) @mfumanelli
- [x] [quicktour.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/quicktour.mdx). @mfumanelli
- [x] [installation.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/installation.mdx). @mfumanelli
## Tutorial section
- [x] [pipeline_tutorial.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/pipeline_tutorial.mdx) @nickprock
- [x] [autoclass_tutorial.mdx](https://github.com/huggingface/transformers/blob/master/docs/source/autoclass_tutorial.mdx) @mfumanelli
- [x] [preprocessing.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/preprocessing.mdx) @nickprock
- [x] [training.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/training.mdx) @nickprock
- [x] [accelerate.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/accelerate.mdx) @mfumanelli
- [ ] [model_sharing.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/model_sharing.mdx) WIP @mfumanelli
- [ ] [multilingual.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/multilingual.mdx) WIP @nickprock

## How-to guides
- [x] [fast_tokenizers.mdx](https://github.com/huggingface/transformers/blob/main/docs/source/en/fast_tokenizers.mdx "fast_tokenizers.mdx") @andreafailla
- [ ] [create_a_model.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/create_a_model.mdx "create_a_model.mdx") WIP @F02934
- [x] [custom_models.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/custom_models.mdx "custom_models.mdx") @Xpiri
- [x] [run_scripts.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/run_scripts.mdx "run_scripts.mdx") @lorenzobalzani
- [x] [sagemaker.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/sagemaker.mdx "sagemaker.mdx") @andreafailla
- [ ] [converting_tensorflow_models.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/converting_tensorflow_models.mdx "converting_tensorflow_models.mdx") WIP @Xpiri
- [ ] [serialization.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/serialization.mdx "serialization.mdx") WIP @F02934
- [ ] [performance.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/performance.mdx "performance.mdx") WIP @machicomio
- [ ] [perf_train_gpu_one](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_train_gpu_one.mdx)
- [ ] [perf_train_gpu_many](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_train_gpu_many.mdx)
- [ ] [perf_train_cpu](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_train_cpu.mdx)
- [ ] [perf_train_cpu_many](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_train_cpu_many.mdx)
- [ ] [perf_train_tpu](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_train_tpu.mdx)
- [ ] [perf_train_special](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_train_special.mdx)
- [ ] [perf_infer_cpu](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_infer_cpu.mdx)
- [ ] [perf_infer_gpu_one](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_infer_gpu_one.mdx)
- [ ] [perf_infer_gpu_many](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_infer_gpu_many.mdx)
- [ ] [perf_infer_special](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_infer_special.mdx)
- [ ] [perf_hardware](https://github.com/huggingface/transformers/blob/main/docs/source/en/perf_hardware.mdx)
- [ ] [big_models](https://github.com/huggingface/transformers/blob/main/docs/source/en/big_models.mdx)
- [x] [parallelism.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/parallelism.mdx "parallelism.mdx") WIP @Xpiri
- [ ] [benchmarks.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/benchmarks.mdx "benchmarks.mdx") WIP @mfumanelli
- [ ] [migration.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/migration.mdx "migration.mdx") WIP @Baelish03
- [ ] [troubleshooting.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/troubleshooting.mdx "troubleshooting.mdx") WIP @F02934
- [ ] [debugging.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/debugging.mdx "debugging.mdx") WIP @nickprock
- [ ] notebooks
- [ ] [community.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/community.mdx "community.mdx") WIP @lorenzobalzani
- [ ] [add_new_model.mdx](https://github.com/huggingface/transformers/blob/b9a768b3ffa80c4c19d024f9f42d5917e7d8109e/docs/source/en/add_new_model.mdx "docs/source/en/add_new_model.mdx") WIP @Steboss89
- [ ] [add_new_pipeline.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/add_new_pipeline.mdx "add_new_pipeline.mdx")
- [ ] [testing.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/testing.mdx "testing.mdx")
- [ ] [pr_checks.mdx](https://github.com/huggingface/transformers/blob/9c9db751e29432e8924624ef44856cd9fa671ef3/docs/source/en/pr_checks.mdx "pr_checks.mdx")