reduce memory use by streaming file to create weight tensors

[mikowals]

Use the new read_bytes function to read checkpoint in pieces so weight tensors can take ownership without copying.

A couple of minor changes made:

• read checkpoint file in Config.init and TransformerWeights.init. Removed the config_init function.
• checkpoint file size is no longer available in main so I added some printing of eqiuvalent output in TransformerWeights init.

[mikowals]

Not sure if anyone has looked at this but my earlier commit was broken. It is fixed now with a few other small changes to work for Mojo 0.7.0.

[tairov]

Thanks for updating compatibility!