HFTracer.trace should use self.graph to be compatible with torch.fx.Tracer
Author: pbelevich
Pull request: yes



# What does this PR do?

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This PR makes HFTracer.trace compatible with [torch.fx.Tracer.trace](https://github.com/pytorch/pytorch/blob/f51a497f92b64df2f6c31a6f4f9662bfd9937160/torch/fx/_symbolic_trace.py#L571) by modifying `self.graph`(which can be accessed later by clients)


## Before submitting
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
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- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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