Tag TOSA specification for 0.21.0 release

Functional changes:
- MUL, add right shift on 32x32 bit multiply
- ARITHMETIC_RIGHT_SHIFT: add rounding
- RESIZE: add float type
- Add int48 to supported number formats
- Add SCATTER operator

Descriptive changes:
- Pseudocode cleanup, usage clarifications
- Fix various rank limitations
- Rewrite the introduction for improved clarity
- Improve the documentation for attributes and inputs
for training profile vs. inference profiles.