Initial tensor shapes should be unknown

[khatchad]

The initial tensor shapes should be unknown. Currently, they are set to the MNIST dataset shape. This makes the current tests pass.

[khatchad]

Resolved by the lattice changes on branch 267-initial-tensor-dimensions-arent-always-accurate. The read_data → do migration removed the MNIST_INPUT placeholder tagging, so initial shapes are now unknown (⊤) rather than hard-coded to MNIST.