doc: add README for partitioning and automatic fallback

Signed-off-by: Bo Wang <wangbo1995ee@163.com>

core/partitioning/README.md

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+# TRTorch Partitioning
+
+TRTorch partitioning phase is developed to support automatic fallback feature in TRTorch. This phase won't run by
+default until the automatic fallback feature is enabled.
+
+On a high level, TRTorch partitioning phase does the following:
+- Segmentation. Go through the set of operators in order and verify if there is converter for each operator. Then, 
+roughly separate the graph into parts that TRTorch can support and parts TRTorch cannot.
+- Dependency Analysis. For every to be compiled operator there is a "complete dependency graph", which means that 
+every input can to traced back to an input as Tensor or TensorList. Go through all segments after segmentation then 
+  do dependency analysis to ensure that there are only Tensor/TensorList inputs and outputs for TensorRT segments.
+- Shape Analysis. For each segments, figure out the input and outputs shapes starting from the provided input shape 
+from the user. Shapes can be calculated by running the graphs with JIT.
+- Conversion. Every TensorRT segments will be converted to TensorRT engine. This part is done in compiler.cpp, but 
+  it's still a phase in our partitioning process.
+- Stitching. Stitch all TensorRT engines with PyTorch nodes altogether. 
+
+Here is the brief description of functionalities of each file:
+- PartitionInfo.h/cpp: The automatic fallback APIs that is used for partitioning. 
+- SegmentedBlock.h/cpp: The main data structures that is used to maintain information for each segments after segmentation.
+- shape_analysis.h/cpp: Code implementation to get the shapes for each segments by running them in JIT.
+- partitioning.h/cpp: APIs and main code implementation for partitioning phase.
+
+### Automatic Fallback
+To enable automatic fallback feature, you can set following attributes in Python: 
+```python
+  import torch
+  import trtorch
+
+  ...
+  model = MyModel()
+  ts_model = torch.jit.script(model)
+  trt_model = trtorch.compile(model, {
+    ...
+    "torch_fallback" : {
+      "enabled" : True,
+      "min_block_size" : 1,
+      "forced_fallback_ops": ["aten::foo"],
+    } 
+  })
+```
+- enabled: By default automatic fallback will be off. It is enabled by setting it to True. 
+- min_block_size: The minimum number of consecutive operations that must satisfy to be converted to TensorRT. For 
+example, if it's set to 3, then there must be 3 consecutive supported operators then this segments will be converted.
+- forced_fallback_ops: A list of strings that will be the names of operations that the user explicitly want to be in 
+PyTorch nodes.
+
+To enable automatic fallback feature in C++, following APIs could be uses:
+
+```c++
+#include "torch/script.h"
+#include "trtorch/trtorch.h"
+
+...
+auto in = torch::randn({1, 3, 224, 224}, {torch::kCUDA});
+
+auto mod = trtorch::jit::load("trt_ts_module.ts");
+auto input_sizes =  std::vector<trtorch::CompileSpec::InputRange>{{in.sizes()}};
+trtorch::CompileSpec cfg(input_sizes);
+cfg.torch_fallback = trtorch::CompileSpec::TorchFallback(true);
+cfg.torch_fallback.min_block_size = 1;
+cfg.torch_fallback.forced_fallback_ops.push_back("aten::foo");
+auto trt_mod = trtorch::CompileGraph(mod, cfg);
+auto out = trt_mod.forward({in});
+```