Make Element requirement in TensorArrayProtocol.init a conformance.

[bgogul]

Related to https://bugs.swift.org/browse/TF-542

[rxwei]

Do you ever need to call init(_handles:) on an array of type-erased _AnyTensorHandles? If so, this change would make that impossible.

[rxwei]

The Google Swift Style Guide recommends no whitespaces before colons in a conformance constraint.

Suggested change

	init<C: RandomAccessCollection>(_handles: C) where C.Element : _AnyTensorHandle
	init<C: RandomAccessCollection>(_handles: C) where C.Element: _AnyTensorHandle

[bgogul]

Do you ever need to call init(_handles:) on an array of type-erased _AnyTensorHandles? If so, this change would make that impossible.

I have not needed it for the implementation of LazyTensor, but there is one use case in the test. That is why I made the following change:

+copy(of: TensorHandle<T>) -> TFETensorHandle
-copy(of: TensorHandle<T>) -> _AnyTensorHandle

Without the changes in this PR, I was not able to initialize a TensorGroup with an array of specific type, e.g., [TFETensorHandle] or [LazyTensor]. Is there a better way to achieve this?

[rxwei]

A [TFETensorHandle] should be directly convertible to a [_AnyTensorHandle] because it's a subtype.

[bgogul]

A [TFETensorHandle] should be directly convertible to a [_AnyTensorHandle] because it's a subtype.

Ah, that is right. The problem that I faced was with using an ArraySlice<TFETensorHandle> as argument to init(_handles:_).

[rxwei]

I see. Yeah, variance only works with a few stdlib types in Swift such as Array.