* Add CheckpointReader and corresponding C APIs.

* Add essential components of SavedModel format loading.

* Add checkpoint reading for SavedModel format loading.

* Revise customized json converters.

* Add support for loading models from python.

* Fix the duplicated weights in Keras.Model.

* Add alexnet loading test and check for loaded weights.

* Fix ci error caused by branch merge.

* Resolve the comments and errors.

* Fix the stucking of training when loading model.

* Fix the stucking of training when loading model.

* fix intptr.

---------

Co-authored-by: Haiping Chen <haiping008@gmail.com>

* Refactored DisposableObject

* Added different build directory for TensorflowNET.Examples.GPU

* _FetchHandler: Switched to NPTypeCode

* gfile.cs, Walk(...): Handle case when directory top doesn't exist.

* Tensor.Creation: Perf-opted when creating tensor from NDArray of string

* Graph.cs: refactor and added docs

* Tensor.Creation.cs: perf-ops

* Tensor.Explicit.cs: perf-ops

* Copied globals.regen from NumSharp

- Added supported_numericals_TF_DataType

* Tensor perf-ops and cleanup, Revamped dtypes.cs, some renames.

- Cleanup and docs to all Tensor.cs files
- Changed all uses of System.Convert to NumSharp.Utilities.Converts
- Added all missing types in dtypes.cs
- Renamed tensor.Data<T> to tensor.ToArray<T>, added obsolete message
- Renamed tensor.Data() to tensor.BufferToArray(), added obsolete message
- Made GraphKeys to use const string instead allocating strings at every use of GraphKeys.

* Tensor: Added guards for explicit casts.

* Tensor: Added explicit cast to string

* Tensor.ToArray<T>(): Added support for cases when tensor is scalar.

* Tensor.BufferToArray(): Fixed to use long instead of int.

* TensorShape: Revamped and documented.

* BaseSession: Added Session.run(ITensorOrOperation fetche, params FeedItem[] feed_dict)

* Tensor: renamed _dtype to _override_dtype

- Fixed all locations _dtype is used incorrectly.

* Fixed unit tests

* Tensor.Operations: Reverted commit

* DisposableObject: sorted internal_dispose to properly handle Dispose() calls

* Tensor.DisposeUnmanagedResources: Nullify _handle after delete.

* TensorShape.this[...]: fixed guard check.

* DisposableObject #362

* removing unused namespaces

* implementing Tensor to scalar and scalar to tensor conversions