[FX] refactor the fx path in compile function

[frank-wei]

Description

Separate the FX lowering process to a single function compile to align with existing ts compile.

Type of change

Please delete options that are not relevant and/or add your own.

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
• I have added the relevant labels to my PR in so that relevant reviewers are notified

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[yinghai]

Other general comments:

1. Using [FX] instead of [fx2trt]. We should drop such reference in oss.
2. Once or is accepted, using squash and merge so that it's checked in as one commit to avoid littering in the commit history with a lot of intermediate commits in the PR.

[frank-wei]

dynamic_batch is added to differentiate two cases: with or w/o dynamic shape on batch dim (dim=0). cc @wushirong. I keep the dynamic_batch=True as default value so it will not change the previous behavior in production. Please have a review.

[yinghai]

What's the use case of w/o dyanmic shape on batch dim with explicit_batch_dimension=True? What's the different in terms of behavior in TensorRT? Basically, if I have explicit_batch_dimension=True while all my input dims are positive, how does TRT interprets it?

Maybe a question to @narendasan too.

[frank-wei]

Here is my understanding, firstly, explicit_batch_dimension=True will become the default in next year and there is no explicit_batch_dimension=False(implicit) mode.
Secondly,

if I have explicit_batch_dimension=True while all my input dims are positive, how does TRT interprets it?

TRT will treat it as fixed shape for any future input. And that is what I tested for all the torchdynamo benchmarks

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[wushirong]

Add a description for the new field 'dynamic_batch', and I am a bit unsure whether we should set it default to true.

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[frank-wei]

Add a description for the new field 'dynamic_batch', and I am a bit unsure whether we should set it default to true.

addressed and set it to True as default.