Change Dense.bias type to Optional

[efremale]

TrivialModelTests.swift test is currently failing.

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[saeta]

Could you add a test to verify that this will work with CopyableToDevice (if there isn't one already)? Thanks!

[dan-zheng]

CI fails for a test that seems related to key path iteration:

Test Case 'TensorVisitorPlanTests.testMask' started at 2020-12-09 18:57:37.697
/swift-apis/Tests/x10/TensorVisitorPlanTest.swift:55: error: TensorVisitorPlanTests.testMask : XCTAssertEqual failed: ("[true, false, true, false]") is not equal to ("[true, true]") -
Test Case 'TensorVisitorPlanTests.testMask' failed (0.001 seconds)

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Closing for now because optional differentiation isn't fully supported (SR-13700), so usability wouldn't be great (if users can't differentiate through dense.bias ?? default nil coalescing).

Investigating the test failure on this branch would be a great step towards a smooth migration to using Optional differentiation in deep learning APIs and models (e.g. ResNet) instead of the current, less efficient workarounds!