[WIP] [doc] performance/scalability revamp (#15723)

* [doc] performance/scalability revamp

* link the new docs

* no :

* mixed precision

* work on the first doc

* expand the main doc

* Trigger CI

* style

* revamp single GPU training section

* work on training performance

* remove files not used anymore or will be added later

* final touches

* fix rebase

* Add hardware section to toctree

* fix toctree again

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* remove `fast_tokenizers` entry that was copied in rebase

* add warning about DP vs DDP

* remove todo

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* fix missing closure of codeblock

* Update docs/source/en/perf_train_gpu_many.mdx

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* sync with #16860

* update toc

Co-authored-by: leandro <leandro.vonwerra@spoud.io>
Co-authored-by: Leandro von Werra <lvwerra@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/en/_toctree.yml

@@ -1,4 +1,4 @@
-- sections: 
+- sections:
   - local: index
     title: 🤗 Transformers
   - local: quicktour
@@ -60,11 +60,9 @@
   - local: serialization
     title: Export 🤗 Transformers models
   - local: performance
-    title: 'Performance and Scalability: How To Fit a Bigger Model and Train It Faster'
+    title: Performance and scalability
   - local: big_models
     title: Instantiating a big model
-  - local: parallelism
-    title: Model Parallelism
   - local: benchmarks
     title: Benchmarks
   - local: migration
@@ -83,6 +81,12 @@
     title: "How to add a model to 🤗 Transformers?"
   - local: add_new_pipeline
     title: "How to add a pipeline to 🤗 Transformers?"
+  - local: perf_train_gpu_one
+    title: Training on one GPU
+  - local: perf_train_gpu_many
+    title: Training on many GPUs
+  - local: perf_hardware
+    title: Custom hardware for training
   - local: testing
     title: Testing
   - local: pr_checks