[DTensor] implement layout converter

[YuliangLiu0306]

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🚨 Issue number

#3067

📝 What does this PR do?

LayoutConverter mainly has three main functions:

1. Search an efficient transform path for the given pair of source_layout and target_layout.
2. Estimate the converting cost for the given pair of source_layout and target_layout.
3. Apply the layout converting path to redistribute a tensor from source_layout to target_layout.

To search the transform path, we use a heuristic algorithm which could be simplily describe as:

• Step1: Generate all one-step transform sequences from source_layout.
• Step2: Pick the 'best' layout following the heuristic function.
• Step3: Repeat above steps until the source layout transform to target layout.

To estimate the total converting cost, we implement a method to estimate the communication cost of each communication operation under the instruction of CommSpec. Then, we just need sum the costs of CommSpec in the transform_sequence up to get the total converting cost.

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[github-actions]

The code coverage for the changed files is %.

Click me to view the complete report

Name                                                      Stmts   Miss  Cover
-----------------------------------------------------------------------------
colossalai/tensor/d_tensor/layout_converter.py              200     24    88%
colossalai/tensor/d_tensor/utils.py                          38      7    82%
tests/test_tensor/test_dtensor/test_layout_converter.py      97      1    99%
-----------------------------------------------------------------------------
TOTAL                                                       335     32    90%

[FrankLeeeee]

Looks good to me.