[flake8](http://flake8.pycqa.org) testing of https://github.com/apache/incubator-tvm on Python 3.8.0

$ __flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics__
```
./apps/vta_rpc/start_rpc_server_to_tracker.py:18:18: E999 SyntaxError: invalid syntax
PROJROOT="$( cd "$( dirname "${BASH_SOURCE[0]}" )/../../" && pwd )"
                 ^
```

* [Relay][Frontend][TFlite] Add parses support for SLICE

* TFlite 1.13: convertor gives nonsense output when size[i]==-1
* TF parser: SLICE need fixing for size[i]==-1 -> gives wrong output
  bcs of indices

* Set end[i] = input_tensor_shape[i] as suggested in PR review

* Add another test to cover size=-1 case

https://github.com/actions/checkout/releases

* Update topi.cc

fix topi.nn.global_pool layout="NHWC"

* add topi.nn.global_pool layout=NHWC test

* [REFACTOR][IR] tvm::Expr -> PrimExpr(Primitive Expr)

As part of unified IR, we will need to unify relay::Expr
and the current tvm::Expr under the same base type.

From the techinical point of view. tvm::Expr is a "primitive"
expression that only contains POD types and handles and does
not do life-cycle management.

This PR renames Expr->PrimExpr to clarify that.
We will send a subsequent PR to introduce the base expr class.

* Remove legacy VarExpr and ExprHash/Equal

* [Relay][Frontend][Tflite] Add parses support for unary elemwise ops

* Add generic method to convert unary functions: abs, exp, ceil, floor
  log, sin, cos, sqrt, rsqrt, neg
* Add relevant tests

* Delete excessive underscores as requested in PR review

* Change parameter name as suggested in PR review

* [REFACTOR] relay::Module Def -> TypeDef

The term Def was not very clear about what is the object of interest(could be function def or type def).
Changes the term to TypeDef to be more explicit.

* Update include/tvm/relay/module.h

Co-Authored-By: Wei Chen <ipondering.weic@gmail.com>

Co-authored-by: Wei Chen <ipondering.weic@gmail.com>

* Added 1D pooling to Topi

* Added 1D pooling relay op and tests.

* Added onnx parsing and tests for maxpool1d and averagepool1d

* formatting

* moved partial import.

* Fixed typo.

* [AutoTVM] Use vm compile in extracting task from relay

* update

* restructure vm compiler to reduce task extraction time

* x

* fix

* update doc

* udpate doc

* lint

* [RUNTIME] Fix windows build after the latest dso module change.

Switch to shared_ptr to get around a problem in latest MSVC.

* [CI] Add github action for win mac build.

Co-authored-by: Tianqi Chen <tqchen@users.noreply.github.com>

* [REFACTOR][IR] Variable -> VarNode

* [REFACTOR][IR] Add/Sub/Mul/Div -> AddNode/SubNode etc.

* [REFACTOR][IR] Min/Max/FloorDiv/FloorMod -> MinNode/MaxNode etc.

* [REFACTOR][IR] EQ/NE/LT/LE/GT/GE/Select -> EQNode/NENode etc.

* [REFACTOR][IR] Add Node suffix to Select/Call/Load/Ramp/Shuffle/Let

* [REFACTOR][IR] Add node suffix to IntImm/UIntImm/FloatImm/StringImm

* [REFACTOR][IR] Add Node suffix to Any, AttrStmt, AssertStmt

* [REFACTOR][IR] Add Node suffix to Store/Provide/Allocate/Free

* [REFACTOR][IR] Add Node suffix to ProducerConsumer

* Fix lint

* style updates, test fixes

* [RUNTIME][DSO] Improve TVMBackendPackedCFunc to allow return value.

Previously the signature of LibraryModule's PackedFunc does not support return value.
This wasn't a limitation for our current usecase but could become one
as we start to generate more interesting functions.

This feature also start to get interesting as we move towards unified
object protocol and start to pass object around.
This PR enhances the function signature to allow return values.

We also created two macros TVM_DLL_EXPORT_PACKED_FUNC and TVM_DLL_EXPORT_TYPED_FUNC
to allow manual creation of functions that can be loaded by a LibraryModule.

Examples are added in apps/dso_plugin_module.
The change to TVMBackendPackedCFunc is backward compatible,
as previous function will simply ignore the return value field.

* address review comments

* Make Relay Keras frontend support networks created using
   Tensorflow (1.13) Keras implementation (tf.Keras)
 * Modify Keras frontend tests to run from a class rather than a
   function based script
 * Adjust Keras frontend tests to run with both 'Keras' and 'tf.Keras'
 * Change "TestKeras.test_forward_merge" to validate instances by
   class name rather than instance type

* Fix typos on Docker image versions that we are currently running
   as part of CI

 * Add version comment in the same pattern for ci_lint image

* As a result of backwards incompatible changes released in pillow 7.0,
   torchvision crashes if you just "pip install pillow", as we do in
   a few places.

 * This patch sets pillow<7 to be installed in Dockerfiles and support
   material as tutorials and documentation.

* Improve commentary for operator fusion.

* Attempt to clarify what well formed checker is doing

* [REFACTOR][IR] Introduce SeqStmt to replace Block

ir::Block was used to represent a sequence of Stmts in the original low-level IR.
The nested ir::Block structure is not really friendly for recursive visits,
especially when the statements are unrolled.

This PR introduce a SeqStmt that directly stores a sequence of statements in an Array container.
The new SeqStmt will be used as a replacement of the original Block structure.

* [REFACTOR] Migrate use of Block to SeqStmt.

* [REFACTOR] Remove Block

* Add more comments per yizhi's comment

* [CONV] Asymmetic padding

* fix lint error

* update for legalize, rocm and cudnn

* add more test cases

* change more symmetric padding

* change conv2d winograd tests according orginal cases

* remove 'alter_op_layout.h' header in bitserial.cc

* Support empty tensor

* Fix schedule

* Refactor

* Minor fix

* Fix pylint

* Merge cpp and python is_empty_shape

Previously we support a limited case of function type deduction and in many places
we have to supply the type twice during set_body_typed (one in the template parameter, another in the lambda signature).

This PR improves the deduce function by enablng automatic function signature deduction.

```
TVM_REGISTER_GLOBAL("sub")
.set_body_typed([](int x, int y) -> int { return x - y; });
```

Unfortunately, because of template conflict, we can not support the original case
where both type signature and lambda are supplied through set_body_typed.

This PR refactors the existing regsitration to the new style.

Rationale: printer is a common infra that is shared across all nodes.