Cleanup pass that relies on IR invariants instead of re-checking
already-guaranteed conditions. No new features; this is a
consolidation/cleanup pass only.
## Changes
- **docsifier (`python_doc_printer.cc`)**: the `ExprStringDoc` escape
scope always wraps the printer's fixed in-memory `ostringstream` sink,
which never short-writes and never enters a fail state. Drop the
streambuf-general short-write reporting in `xsputn`, the ctor `good()`
ICHECK, the dtor `rdstate`/`setstate` dance, and the redundant
post-render `good()` ICHECK; keep the one-line `saw_newline()` contract.
- **relax diagnostics (`well_formed.cc`, `block_builder.cc`)**: the ty
diagnostics test `ty.IsMissing()` on a now non-nullable `Type`, so word
them as "is missing" rather than "is nullptr".
- **relax numeric-gradient tests**: derive the device from the build
target via `tvm.device_from_target` inside the helpers instead of
threading a redundant `dev` argument that duplicates `target` at every
call site; annotate the numpy inputs as `np.ndarray`.
- **target/printer tests**: drop assertions that re-check a condition an
earlier assertion in the same test already guarantees.
Tianqi Chen
453070e1bb